TAIR (Arabidopsis) sequence retreival module

Hi All,

This PR is to add a module which will retreive sequences by AGI identifiers. This module includes tests, some documentation in docstrings, and a sample script which uses the module.

See discussion here: http://lists.open-bio.org/pipermail/biopython-dev/2012-September/009956.html
And here: http://lists.open-bio.org/pipermail/biopython-dev/2012-October/009977.html