Cmsplugin-form-handler

* Add good test coverage, travis CI and Coveralls support
* Update code for clarity, PEP-8 and line-length (80 columns)
* Update documentation for new methods, and a note about Crispy Forms
* Update signature of get_form_kwargs() to include `instance`
* Add `get_form_kwargs()` method to the plugin base class (thanks: hcNick)
* Store state as QueryDict in sessions (thanks: jproffitt)

* Update documentation (thanks: marksweb)
* add CONTRIBUTORS.txt

* Update documentation

* Add `request` to signature of `get_form_class`, `get_success_url`
and `form_valid`.
* Add proper documentation
* Add this `CHANGELOG.txt`