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.
Merge pull request #2014 from jseabold/arima-diff-2

BUG: Fix forecasting for ARIMA with d == 2