allow users to invite new users

Whoosh,

So, this should all work but still needs to be tested well, it's a big change. Also some dutch locales are probably missing (for the invitation mail etc), we should add those.

This might also solve #139 as it gets rid of devises :registerable module in favour of our own endpoint.

![screen shot 2014-08-13 at 01 32 11](https://cloud.githubusercontent.com/assets/749864/3899349/36b0ff8c-227a-11e4-9f02-be9c4d33f24e.png)

fixes #152