Gets modified_time from storage, if available. Moved  common functionality for getting thumbnail and source mod times to get_modified_time in utils.py

Fixes # 180

Since storages' modified_time functions return a datetime and not a timestamp, I changed the utils.py "fromtimestamp" function, which assumed a timestamp and performed the conversion and then did timezone-aware conversion, to "make_time_zone_aware" which assumes a datetime. To deal with the fallback case where a storage doesn't implement modified_time and we use os.path directly, the new utils.get_modified_time converts the timestamp to a datetime and return that for consistency.
Merge pull request #180 from roberts81/master

Use modified_time method provided by Storage interface instead of assuming os.path compatible storages when getting mod times

Conflicts:
	easy_thumbnails/utils.py
Always assume Storage.get_modified_time exists

Since we only support Django 1.4+ we can drop some legacy code.
Faster modtime comparisons for thumbnail_exists

Reworked the thumbnail_exists method to bail faster if something doesn't
add up. Also only check modtimes from storage if it's local (otherwise
rely on the database cached values).
Update changelog