Backport PR #1057: COMPAT: Fix py3 caching for get_rdatasets.

This is against master. I'm assuming there aren't any problems cherry-picking this to the maintenance/0.5.x branch, but if this is undesirable I can make the commit against it.
Backport PR #1071: Update setup.py to fix broken install on OSX
Backport PR #1075: BLD: Fix build issue on some versions of easy_install.

This also needs to be backported to maintenance. Will hold off on merging until we're ready for 0.5.1 to see how we want to handle this.
Backport PR #1077: BUG: Allow 1d exog in ARMAX forecasting.

Closes #1076. Another candidate for backporting.
Backport PR #1089: ENH: exp(poisson.logpmf()) for poisson better behaved.

A little better behaved for big counts.
Backport PR #1117: Update ex_arma2.py

Correcting typo, and leaving maparams and arparams around for later comparison.

https://github.com/statsmodels/statsmodels/issues/1116#issuecomment-26216044
Backport PR #1124: BUG: Fix ARIMA prediction when fit without a trend.
Backport PR #1125: REF/BUG: Some GLM cleanup. Used trimmed results in NegativeBinomial variance.
Backport PR #1149: BUG: Fix small data issues for ARIMA.

This closes #1146 and #1046.

1. We now check to make sure that we have at least one degree of freedom to estimate the problem. If so, then we try the estimation.
   1. Most / all of these estimations will return garbage. We have an extra check that we can estimate stationary initial params. Usually we can't in these cases, so the usual error will be raised here asking to set start_params. This should be enough of a warning to the user that this is "odd." If in the small chance the estimation goes through for a model with 5 observations and 1 degree of freedom, it's on the user then to determine things are no good.
2. We now avoid the problem of maxlag >= nobs happening in the call to AR so this avoids the problem of #1046 that also presented itself as part of #1146.
Backport PR #1157: Tst precision master

some adjustments to test precision.

lower to avoid failures on some machines see #1113

(I also have my initial branch written against maintenance)