A python_binary_create task, to bring pex-building under goal.

Review Request #470 — Created June 9, 2014 and submitted — Latest diff uploaded

jsirois, patricklaw, wickman
This task supports 'compatibiity' specs in python_binary targets, to restrict the interpreters the pex machinery can select from.

Old functionality via 'command' is unchanged.

Decided, on balance, not to backport the compatibility support into the old command pathway. This is for convenience (PythonTask already has machinery for choosing interpreters), and to minimize disruption at Twitter due to introduced bugs.  The only demand for this feature right now is from us at Foursquare, and we can easily switch to goal-based python binary building. And anyway, 'command' will go away "soon".
Manually ran the new echo_interpreter_version binary with various different compatibility settings. 

Ran all unittests, for good measure.