Implement split_html_space_chars helper function
Split class names using split_html_space_chars helper function
Added HTML space characters check content test
Adding .DS_Store to list of ignored files
auto merge of #2032 : brunoabinader/servo/html-whitespace, r=Ms2ger

Specs:
http://dom.spec.whatwg.org/#concept-ordered-set-parser
http://encoding.spec.whatwg.org/#ascii-whitespace

This PR implements the HTMLSpaceCharSplits iterator, used to split a string in a subset of strings separated by valid HTML space characters. Its first usage is upon splitting ```class``` attribute values.

Closes #1840.