Adds the destVincenty algorithm

This change adds the destVincenty algorithm:
http://www.movable-type.co.uk/scripts/latlong-vincenty-direct.html

Updated the README.md to include new usage and examples of both synchronous and asynchronous operation.

I also slightly modified the existing function to:
- make callback optional
- check # of arguments on callback function, and if 3, unpack the result data into the arguments directly -- see the README for example