Make setup.py usable in environments without requests installed

setup.py should never rely on executing a module (including importing from it); this potentially introduces a bootstrapping problem, as is the case here with "requests". Build systems trying to build your package need to act on the install_requires parameter in the setup() function, in order to pull in dependencies, but this can never be reached.

(Reference - http://stackoverflow.com/questions/2058802/how-can-i-get-the-version-defined-in-setup-py-setuptools-in-my-package?answertab=votes#tab-top )