Split into library and sample project

Makes it easier to add just reference the play billing project, rather than adding the files to your own project.
Also removed the obj folders from git as they shouldn't be there.

Main commit is https://github.com/danzel/play.billing.v3/commit/8922acb80bec88d96d2ea4e2cfbac6e0c8b0e488

(Thanks for the lib btw, saves a lot of work!)