BUG: Fixed xrange probem in MixedLM profile; fixed complex number problem in MixedLM Hessian

Small but important bug fixes:

* Error in profile_re, which previously had no test.  I fixed the error and added a test.

* Basic test coverage for EM

* Fix the issue that was producing a warning about complex numbers.

I think the super call `super(MixedLM, cls).from_formula` should be at the end, and the exog_re put into the kwargs.
Then the exog_re should/would go through the extra arg/kwd processing and we don't need to replicate the last part here that attaches k_re, nparams, ... (which are duplicated from `__init__` AFAICS, and would be handled there)