Return falsy Thumnbnail if no source image is supplied

1. Raise ValueError with surpressed attribute on Thumbnail requiring metadata
2. Fix human readable output of Thumbnail class representation __repr__
3. Added test to validate error surppression in template and raises correctly anywhere else
Merge branch 'master' into fix-error-not-surpressed

Conflicts:
	thumbnails/files.py
Fixed test for AttributeError
Separate test for thumbnails without source
Added "set_quality" image processor.
Mimic django ImageField's behaviour
Minor fix with error raised
Merge pull request #41 from SeiryuZ/fix-error-not-surpressed

Return falsy Thumbnail if no source image is supplied
Merge branch 'master' into post-processors
Make post_processor.optimize accept kwargs

This is done so that post_processors function could freely define
arguments that are needed for the function to perform correctly.