Fixes #135, Do not require GitHub Organization for orgs created via Admin

### Do not require GitHub Organization for orgs created via Admin

This is a `github_org_requirement` branch version of an awesome refactor submitted by @pschlund (#158), covering a few bases:

* No longer requires GitHub org by default for new organizations (e.g. those created in Admin)
* Continues to require GitHub org for projects submitted on [the project submission page](http://www.codemontage.com/organizations/new)
* Adds appropriate tests for public project submission

The UX on this could be improved across the site (see #170), but it works!
![codemontageorgsubmissionpublic_githubreq-5](https://f.cloud.github.com/assets/226228/2059499/c4ccf04e-8bb4-11e3-9dc1-18507097bbe6.png)