BeerXML Support

This is the first set of commits, I'll be doing some tidying up tonight to sort some things out, but I figured I'd push this so you can take a look first.

* Add a Quantity class which supports conversion between different weight types
* Allow Weight to use a string constructor which creates a Quantity class then converts this back to ounces as the Weight default.
* Add a basic BeerXML parser that uses XPath lookup (there's no DTD for BeerXML that I can find)
** Returns an array of recipes
* Add an "Open" option to the RecipeListFragment that will allow XML files to be selected
** This will determine that the file is a BeerXML file then opens the BeerXML parser. 
** This allows us to support more filetypes in the future.
* Add a ParseXML class that has a helper that identifies the recipe type (either via File path or InputStream)
* If a list of recipes comes back, add them all to the Datastore.
* if one recipe comes back, show it, otherwise just refresh the list.
What do you see as the difference between Dry Hop and Secondary? Should we show both options to the user?