BUG: Make sure RE names are properly handled in MixedLM.

Fixes the issue mentioned on the mailing list about RE names. This needs tests. It would be great if someone else wrote those tests, because I don't know what the correct output is necessarily supposed to be. This matches the approach in the code before I refactored it so that the wrappers work, there was a thinko, and the stuff in from_formula needs to be handled differently for now unfortunately. MWE

    import pandas as pd
    import statsmodels.api as sm
    import statsmodels.formula.api as smf

    data = sm.datasets.get_rdataset('dietox', 'geepack').data

    data['constant'] = 1
    mod = sm.MixedLM(data['Weight'], data[['constant', 'Time']],
                     groups=data['Pig'], exog_re=data[['constant', 'Time']])
    modf = mod.fit()
    print modf.summary()
    print modf.params


    mod  = smf.mixedlm('Weight ~ Time', data, groups = 'Pig', re_formula = '~Time')
    modf = mod.fit()
    print modf.summary()
    print modf.params

Closes #2099.