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.
Can we make this a method variable? Otherwise I think GSON will try to serialize the 'quantity' member variable to storage.