Singleton and Localization

Thanks for this great project!

I've made a couple changes that I hope you'll like. 

The first main change is I made Harpy a singleton with a `sharedInstance` method. This allows the `appID` and `alertType` to be set without having to modify the constants header. In-fact I removed this header file completely. The reason for this is that having to modify files in the repo make it difficult to use git submodules. This should fix #18.

The second change moves the strings into a localizable file. This also fixes #16 but in a  slightly different way.