add api.py to all subdirectories

see https://groups.google.com/group/pystatsmodels/browse_thread/thread/9cdfd56fd659659a?hl=en

I think it will be possible to do it in 0.4.1 without breaking backwards compatibility if we don't remove imports from the __init__.py

Remove all imports from __init__.py for release 0.5.0

I haven't looked at subdirectories at lower levels, eg. genmod.families which doesn't have code that isn't needed by GLM. An api.py is really not necessary in this case, unless we add generic functions in there (e.g. pre-specified transformation that I want also for other models and methods)

One other exception, directories for only internal use have an empty __init__.py already, e.g.

statsmodels\base has only the test runner in the __init__.py
statsmodels\tsa\base completely empty
statsmodels\compatnp completely empty

Start with the linked Google Groups discussion and inventory statsmodels subdirectories plus each init.py, including statsmodels/base, statsmodels/tsa/base, and statsmodels/compatnp. Determine which packages need api.py and preserve the stated 0.4.1 compatibility before checking that 0.5.0 removes the relevant init.py imports.