Add CoordinateMapper and unittest

A bit belated, but now that the new SeqFeature type has been established, I am finally submitting a pull request for my Coordinate Mapper. Many thanks to Reece Hart for creating the original code.

Features: 
* Transformation of positions between genomic, CDS, and protein coordinates.
* Includes methods for converting locations to and from HGVS conventions.
* Genbank locations can be parsed with SeqIO.

An example of its usage is on the cookbook: http://biopython.org/wiki/Coordinate_mapping
The example code can also be found in this gist: https://gist.github.com/lennax/10600113

Passes unit test on my Ubuntu 12.04 system.

Please let me know if you have any questions or comments.