[COOK 1063] teach python_pip about requirements files

Changes implemented
---------------------------
- add **:install_requirements** action to pip resource
- get path to requirements file from name attribute

Related Ticket
-----------------
http://tickets.opscode.com/browse/COOK-1063

Related Pull Requests
--------------------------
* _#18 [COOK-1715] Add user and group to python_pip_

This request depends on #18 being merged as it requires the refactorings implemented therein.