mockfs: ensure that makedirs() raises on existing directories

os.makedirs() should raise OSError if it is told to create a
directory that already exists.

Closes #6

Reported-by: Jacob Walker <jacob@tortugas-llc.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Makefile: allow overriding the default prefix via the environment

Allow the MOCKFS_PREFIX environment variable to override the default
prefix.  Adjust the default python site-packages directory so that it
uses the same directory structure as is used by the current python.

Signed-off-by: David Aguilar <davvid@gmail.com>