Make mako template lookups pluggable.

This code adds the ability to add Mako template lookup directories on
the fly, allowing third party add-ons to contribute their own Mako templates.
A new API function for registering Mako templates is introduced::

    from edxmako import add_lookup

    add_lookup('main', '/path/to/templates')

    # Or, specify a package to lookup using pkg_resources.  This will
    # add the 'templates' directory inside the current package:
    add_lookup('main', 'templates', package=__name__)