Add a new draw keyword which will allow you to init mapobj without drawing

Patch originally authored by Christer to make it possible to init a pydarn mapobj as a derived class from basemap mapobj without rendering any figures or axis. You need to init the mapobj to do coordinate transforms and the like, but without this patch you end up creating a blank figure.

Discussion reference:
https://groups.google.com/forum/#!topic/davitpy/OIS0slvED9Q

Please keep this merge open for a bit. I'm going to write a __main__ section with a quick test and look to extend the patch to provide a reusable draw method to polish up the interaction a bit more.