Add Polyline & Alpha support

[warrenc5]

Hi, great work btw, thanks for this plugin.

At the moment I can draw a polygon, but I would like to draw an outline of a route.

I just add the support to the Mapbox.java similar to Polygons, copy and paste.

It worked on Android.. And it works on IOS.

Is looks like it's supported by the libMapbox.a file, because the header files are there.

The only thing I would wish for is polyline alpha so you can read the street names underneath.

It was that easy for me to add it.

Cheers.