Added test case for post_processors
Modified tests and added comments

1. Removed unneeded test for now
2. Added comments about subprocess.call and output redirection
Modified note on output redirection on post_processor.optimize
Merge branch 'master' into test-post-processors

Conflicts:
	thumbnails/tests/settings.py
Merge pull request #48 from SeiryuZ/test-post-processors

Added test case for post_processors