Add WP8 support as discussed in issue [#21]

Hi,

I've finished my work on adding basic WP8 support. A few remarks:

- All three platforms support the startLocationTracking/stopLocationTracking functions. This name was already used for iOS, for WP8 it was added obviously, and for Android I simply changed the Java code to reroute startLocationTracking to startGeoPushes and stopLocationTracking to stopGeoPushes. This makes the API more uniform.

- I've added capability 'ID_CAP_LOCATION' to the WP8 build because they are needed for location tracking. Developers can remove the capability if they don't need those methods.

- When the app is started, there is a check for NotificationService.LastPushContent which is always an empty string. It may be because I'm missing some configuration but without knowing the internals of the Push SDK it's impossible to tell. So what you don't get is the payload of the last push when you open the app. I can do without it, but the plugin is more useful when this is fixed some day.

I've tested all three platforms after these changes and it all works as expected. There is a demo app here if you want to verify it (it's for Telerik AppBuilder): https://github.com/Telerik-Verified-Plugins/pushwoosh-phonegap-3.0-plugin-DemoApp

I hope you like the changes and are willing to merge it into your repo.

Thanks,
Eddy