Documentation in place, 2.3.0

This PR lands a whole lot of changes.

 - Documentation has been added and will be on readthedocs at some point in the near future
 - All methods in all classes now have JSDoc docblocks
 - Numerous missing methods have been added:
  - `getServerStats()`
  - `getServerStatus()`
  - `getShortUrl()`
  - Plenty of helper methods for URL-building.
 - Some code have been refactored to make it more flexible and future-proof
 - An `ImageUrl`-class have been introduced, extending `Imbo.Url`.