Adding a missing getsitepackage function to install in dev mode

You can't install Bokeh in a virtualenv because the lack of getsitepackages()
This is an open bug at venv: https://github.com/pypa/virtualenv/issues/355
And pretend to offer a workaround to fix our issue until it is fixed upstream (venv): https://github.com/ContinuumIO/bokeh/issues/378