BUG: Fix failed import of mlab.

Fix imports in the numpy/oldnumeric/mlab.py module.  The problem was
that the module used imports of the form

from . import compat

which do not work as '.' is apparently undefined when the module import
is attempted. The fix is ugly, but it works and the oldnumeric module is
going away in 1.9.

Closes #3803.
Merge pull request #3805 from charris/fix-mlab-import

Fix mlab import