Fix issue #219 using same approach as has now been committed to graphite-web.

This overrides my previous pull request #221. Instead, this provides a workaround for issue #219 using same approach as @brutasse in [graphite-web commit d231f2f7f](graphite-project/graphite-web@d231f2f7f). That approach allows overriding of the install location with ```GRAPHITE_NO_PREFIX=1 pip install [--install-option=--prefix=...] ...```. This pull request makes carbon consistent with graphite-web with respect to installation procedure.

I have tested this on OS X and Ubuntu using virtualenv, but I have not attempted any additional testing, nor have I attempted to build the respective distro packages.