Closes #1562. This restricts ARIMA to maximum of 2 differences. Thinking about the general case made my head hurt more than I want for this release.
STY: Cleanup imports
ENH: Cleanup unintegrate. Add unintegrate_levels
BUG: Fix ARIMA.forecast for d > 1.
TST: Test forecast with ARIMA d > 1.
TST: Test predict for ARIMA with d > 1
BUG: Fix prediction for ARIMA d > 1. Closes #1562.
Coverage increased (+0.02%) when pulling 26cd6fc on jseabold:arima-diff-2 into 2d4f835 on statsmodels:master.