Rewrite /dist-packages/ to /site-packages/ if and only if /site-packages/ is in `sys.path`.

This avoids pip-accel to break Ubuntu compatibility. Fixes issue #28 when installing system wide.
Rename optional keyword argument of install_binary_dist()
Merge pull request #29: Compatibility w/ system wide Debian based installs

During the merge I changed the implementation because I don't want the
pip-accel binary cache to contain /dist-packages/ pathnames: the binary
cache should adhere as much as possible to the Python defaults, not the
Debian defaults, because the main (intended) use case for pip-accel is
still to bootstrap virtual environments.