Provide default resources for pin markers

[cyrilmottier]

Polaris is intended to make the developer's life easier. Providing default resources is a great way to avoid painful photoshopping for developers.

The idea here is to bring and enhance the MapPinDrawable concept introduced in a previous library of mine called GreenDroid (see https://github.com/cyrilmottier/GreenDroid/blob/master/GreenDroid-GoogleAPIs/src/greendroid/graphics/drawable/MapPinDrawable.java for more info)

[f2prateek]

This is a simple port from https://github.com/cyrilmottier/GreenDroid/blob/master/GreenDroid-GoogleAPIs/src/greendroid/graphics/drawable/MapPinDrawable.java. Updating the base assets for Polaris would probably be a good idea to fit in with the newer styles.