BUG: Make sure RE names are properly handled.
EX: Add MixedLM example notebook.
ENH: Guard against non-PD covariance.
TST: Test wrappers and names for MixedLM
EX: Cleanup for refactor. Add R code.
EX: Add title to notebook.
REF: Print statements -> functions
DEV: Update for IPython 3.X. Titles can be markdown
DOC: Add MixedLM example to landing page.
Merge pull request #2097 from jseabold/fix-mixedlm

BUG: Make sure RE names are properly handled in MixedLM.
ENH: Add correct logdet for pos. def. matrices.
BUG: Deprecate logdet with incorrect pos. def. check. Closes #2087.
REF: Do not use deprecated function internally.