Added info on advanced usage with ignore

Couldn't find documentation on how to use the ignore option. Having figured it out I thought it would be useful for others if it was in the readme. Thanks for making this plugin!
Rename on_selection_modified method
Refactoring with 2to3 tool
Fix relative import
Remove begin_edit() and end_edit()
Fix bytes to string error
Minor editing to readme update from benfrain.
Merge branch 'develop' of git://github.com/nDmitry/sublime-csslint into ST3
conditional import for thread
* PEP8 formatting (indentation mostly)
* conditional import of thread to support python 2 and 3
* added test css file (no actual tests written)
conditional importing of statusprocess and formatting issues
* refactored code and got rid of asyncprocess and statusproces.py.
* plugin is compatible with ST3 now - pending windows testing.
* added a big test file for testing.
renamed tests to test_files, since these are test css files, not actual tests.
removed reference to asycprocess
added required subprocess import
added messages, updated changelog