Added SplashScreen plugin

http://github.rim.net/webworks/BB10-WebWorks-Cordova/issues/90
Implemented hide()

Note to tester: You need to specify this in config.xml and make sure you include a splash image:
```xml
<rim:splash src="some-image.jpg" />
```
```xml
<preference name="AutoHideSplashScreen" value="false" />
```
See Apache [docs] (http://cordova.apache.org/docs/en/2.5.0/cordova_splashscreen_splashscreen.md.html#hide) for SplashScreen plugin

@bryanhiggins @jkeshavarzi Code review please.