Update docstring for Arg
Add Missing value that parser methods return if value not on request

This allows a parser to differentiate between a missing value and
when None is passed explicitly

Implements the behavior described in #9

The FlaskParser is the only parser using this API
Bump dev version and update CHANGELOG
Modify all parsers to return Missing when appropriate
Fix version number and update docs
Skip Django tests on py26

Newer versions of Django are incompatible with py26
Fix bug that caused ValueError when parsing json
Add `validate` parameter to Parser.parse for validating across args

Possible solution to #10
Update docs
Add flake8
Fix flake8 on travis

Also, fix header underlines in CHANGELOG
Update CHANGELOG and bump version