TST: Test missing keyword in ARMA
BUG: ARMA model discards missing keyword. Closes #1343.
Merge pull request #1344 from jseabold/arma-missing-kw

BUG: Use missing keyword given to ARMA.
STY: Pep-8 cleanup.
Merge pull request #1346 from jseabold/pep8cleanup

STY: PEP-8 Cleanup