Singleton and Localization

Thanks for this great project!

I've made a couple changes that I hope you'll like. 

First 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.

The second change moves the strings into a localizable file. This also fixes #16 but in a  slightly different way.