Added a temp workwround for deprecated short ext names.

As we chnaged the order in import trys for short names extensions
(no dot syntax), an extra test was added to the import code for the
occassion when a naming conflict exists. For example, if PyTables
is installed (module name is tables) and the user tries to use
the short name 'tables' instead of 'markdown.extensions.tables'.
Fixes #341.

Of course, this code will get ripped out when the old behavior is
fully deprecated.