TST: Test SVD convergence failure due to bad start params.

TST: Add test
ENH: Check for bad values from start_params for ARMA. Closes #675. Closes #15.

BUG: Include all AR params
STY: Remove comment
BUG: Use correct definition for root checking.
Merge pull request #803 from jseabold/fix-arima-error-msg

ENH: Fix arima error message for bad start_params