ENH: Add api for covstruct. Clear __init__. Closes #1917.

closes #1917
(by email, trying to recover from temporary power outage but browsers don't
work properly for github)

First, we never do `import *` anymore, it's not informative, automatic
tools don't work, and I don't see what we have added to the api.

Second, the content of the subpackage `api` are **not** imported into the
statsmodels.api. We use them to create the next layer of import paths with
content that is not entirely in the top level api.
See `from .stats import api as stats` and similar.