Fix some bugs and linter errors introduced by backfiller

I ran `pyflakes` on the repo and found these bugs:

```
./common/common.py:289: undefined name 'random'
./downloader/downloader/main.py:7: 'random' imported but unused
./backfiller/backfiller/main.py:150: undefined name 'variant'
./backfiller/backfiller/main.py:158: undefined name 'timedelta'
./backfiller/backfiller/main.py:171: undefined name 'sort'
./backfiller/backfiller/main.py:173: undefined name 'sort'
```
(ok, the "imported but unused" one isn't a bug, but the rest are)

This fixes those, as well as a further issue I saw with sorting of hours.

Iterables are not sortable. As an obvious example, what if your iterable was infinite?
As a result, any attempt to sort an iterable that is not already a friendly type like a list
or tuple will result in an error. We avoid this by coercing to list, fully realising the iterable
and putting it into a form that python will let us sort. It also avoids the nasty side-effect
of mutating the list that gets passed into us, which the caller may not expect. Consider this example:

```
>>> my_hours = ["one", "two", "three"]
>>> print my_hours
["one", "two", "three"]
>>> backfill_node(base_dir, node, stream, variants, hours=my_hours, order='forward')
>>> print my_hours
["one", "three", "two"]
```

Also, one of the linter errors was non-trivial to fix - we were trying to get a list of hours
(which is an api call for a particular variant), but at a time when we weren't dealing with a single
variant. My solution was to get a list of hours for ALL variants, and take the union.

backfiller/backfiller/main.py

@@ -147,15 +147,19 @@ def backfill_node(base_dir, node, stream, variants, hours=None, start=None,
 		seconds to prioritise letting the downloader grab these segments."""
 
 	if hours is None:
-		hours = list_remote_hours(node, stream, variant)
+		# gather all available hours from all variants and take the union
+		hours = set().union(*[
+			list_remote_hours(node, stream, variant)
+			for variant in variants
+		])
 	elif is_iterable(hours):
-		pass # hours already in desired format
+		hours = list(hours) # coerce to list so it can be sorted
 	else:
 		n_hours = hours
 		if n_hours < 1:
 			raise ValueError('Number of hours has to be 1 or greater')
 		now = datetime.datetime.utcnow()
-		hours = [(now - i * timedelta(hours=1)).strftime(HOUR_FMT) for i in range(n_hours)]
+		hours = [(now - i * datetime.timedelta(hours=1)).strftime(HOUR_FMT) for i in range(n_hours)]
 
 	if start is not None:
 		hours = [hour for hour in hours if hour >= start]
@@ -168,9 +172,9 @@ def backfill_node(base_dir, node, stream, variants, hours=None, start=None,
 	if order == 'random':
 		random.shuffle(hours)
 	elif order == 'forward':
-		sort(hours)
+		hours.sort()
 	elif order == 'reverse':
-		sort(hours, reverse=True)
+		hours.sort(reverse=True)
 
 	for variant in variants:
 

common/common.py

@@ -8,6 +8,7 @@ import errno
 import itertools
 import logging
 import os
+import random
 import sys
 from collections import namedtuple
 

downloader/downloader/main.py

@@ -4,7 +4,6 @@ import errno
 import hashlib
 import logging
 import os
-import random
 import signal
 import sys
 import uuid