readme, fix default challenge type, starting code style conventions

this pull request contains 
* a small fix to challengetypes.py to add the slug to the marshaling fields, and some as yet unused code to consolidate action types on a per-challenge basis. 
* a first swing at code style conventions, which also includes a new wiki page [here](https://github.com/osmlab/maproulette/wiki/Code-style-conventions). I made maproulette.js, models.py and challengetypes.py compliant as an example. The conventions follow jsbeautify for JS and flake8 for python.