Improved Scaladoc for `Brewer` and `Drink` classes.
Added test and implementation to load a single drink from xml.

re: #11
Moved getting of Xml node value into common helper object.

re: #11
Successfully loading beer and cider from xml.

re: #11
Added tests for loading drinks from file.

This should also cover loading from POST requests.

re: #11
Added test stubs for loading brewers.

Also corrected typos in drinks xml load tests

re: #11
Added brewers sample xml and stub `toXml` for `Brewer` class.

re: #11