Code style changes and some initial documentation

I have done the following

  * Made the code lines a bit shorter
  * Added some newlines to group functions
  * Reordered some functions so get_one always comes before get_all_entries
  * Added some documentation accessible via the `h` function in iex, `h Riak`

There is no change in functionality, and thus the unit tests still pass.

My hope this will make the code easier to grok and make it a bit easier to get started using the module.