Expose pitchEnabled property

[benvium]

By default the user can drag with two fingers (iOS) or three fingers (Android) to adjust the tilt of the map. This PR adds the ability to turn that off.

Added to iOS and Android, and also added to the docs.

NOTE: also fixes a small issue where on iOS scrollEnabled and zoomEnabled couldn't be changed at run-time. Developer forgot to assign the new values in the setters.

Example:

 <MapView
      ...
      pitchEnabled={false}
      ...
      />

[bsudekum]

Looks good, thanks!