Tinycss2

- Fix tests launched by CI
- Parse "--" ident tokens correctly

- Drop Python 2.7, 3.3 and 3.4 support
- Allow leading double dash syntax for ident tokens, allowing CSS variables to be parsed correctly.
Test with PyPy3
- Force tests to parse JSON files as UTF-8
- Clean packaging

- Update documentation

- Don't allow identifiers starting with two dashes
- Don't use Tox for tests
- Follow semantic versioning

- Update for spec changes
- Add a `ast.WhitespaceToken.value` attribute to `ast.WhitespaceToken`
- Breaking change: CSS comments are now preserved as `ast.Comment` objects by default. Pass `skip_comments=True` to parsing functions to get the old behavior
- Breaking change: Top-level comments and whitespace are now preserved when parsing a stylesheet, rule list, or declaration list. Pass `skip_comments=True` and `skip_whitespace=True` to get the old behavior
- Test on Python 3.4 and PyPy3
- Set up continous integration on Travis-CI

- Fix `ast.HashToken` starting with a non-ASCII character
- Fix `repr` on AST nodes