TST: fix _infer_freq for pandas .14+ compat

pandas 0.14 fixed a bug with DatetimeIndex's
freqstr attribute. Previously relied on
having the freqstr attribute implying that freqstr
was not none.
Merge pull request #1872 from TomAugspurger/infer_freq_pandas_break

BUG: fix _infer_freq for pandas .14+ compat closes #1822
BUG: add _ctor_param to TransfGen distributions

so that they can be frozen with scipy >= 0.14.
see scipy gh-3852, statsmodels gh-1865
Merge pull request #1883 from ev-br/frozen_transf

BUG: add _ctor_param to TransfGen distributions closes #1864
Fix: Add compat code for pd.Categorical in pandas>=0.15
TST: corr_nearest_factor, add test for sparse svds, objective values
Merge pull request #1884 from JanSchulz/cat_fixes

BUG: Add compat code for pd.Categorical in pandas>=0.15
Merge pull request #1885 from josef-pkt/corr_nearest_factor_1850

TST: corr_nearest_factor, more informative tests
BUG pandas 0.15 compatibility in grouputils labels
Merge pull request #1886 from josef-pkt/BUG_grouputils_pandas_compat

BUG pandas 0.15 compatibility in grouputils labels
ENH: add tools.testing module for asserts (numpy, pandas compat wrapper)
TST: pandas compat: change asserts (failing with pandas master for 0.15)
TST test_corrpsd Test_Factor: add noise to data
Merge pull request #1888 from josef-pkt/corr_nearest_factor_1850_4

TST test_corrpsd Test_Factor: add noise to data
Merge pull request #1887 from josef-pkt/BUG_pandas_compat_asserts

ENH/TST: pandas compat asserts, new statsmodels.tools.testing
BUG: fix ytick positions closes #1561
Merge pull request #1889 from josef-pkt/BUG_multicomp_plot_1561

BUG: fix ytick positions closes #1561
BUG: MultiComparison use array for groups closes #1890
BUG fix more ix indexing cases for pandas compat closes #1891
TST: add tukeyhsd/MultiComparison test with pandas.Series