Made the distinction between development and production environment clearer
Added two constants BeeHub::ENVIRONMENT_PRODUCTION and
BeeHub::ENVIRONMENT_DEVELOPMENT which values should be used to set the
environment variable APPLICATION_ENV (in Apache) to distinguish between these
possible environments
User information are now properties of the user file

The 'user file' is the file which is a 'principal' revering to a user. For example: /user/niek
* To update account information, like e-mail address or password, you can set the appropriate properties on the user file
* POST requests are no longer accepted by the user file. This was used before to change account information, but this is not needed anymore
* The user profile page (returned on a GET request on the user file) is now changed so it uses PROPPATCH to update the information (was POST before).