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.
Update usage.rst

added guide for how to access dimensions. Took me ages to figure it out so I thought it would be nice if it appeared somewhere in the guides.
Allow {% thumbnail %} 'size' to be an alias name. See #248.
 - could extend to allow variables, not just hardcoded  strings as aliases
 - not sure what needs to be done to take alias 'target' into consideration
Make thumbnailer filter accept a relative_name

Fixes #261
Test that missing files act as expected

Refs #183
Some changes for most recent version of tox
Merge remote-tracking branch 'DrMeers/thumbnail_tag_aliases'
Alter some of DrMeers alias code
Remove a redundant step of Aliases.get
Update CHANGES and bump number for release
Improve engine.generate_source_image

Firstly, don't require a close attribute on the source file since not
all file-like objects provide this method. This fixes #114 and refs #259

Secondly, ensure we try to re-open (falling back to seek(0)) between
multiple source generators. Fixes #250
Don't assume the file can't be closed just because it couldn't be reopened.
Fail silently if the source file can't be closed
Prove that getting dimensions of a cached thumbnail still works
Removed verbose storage object from function save_thumbnail
Fixed: sending signal thumbnail_created after having created highres thumbnail
Bug fixed: files in high resolution (@2x) are not deleted
Added signal handler which optionally optimizes thumbnails using external programs
Fixed a typo
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