Fix issue #1993
ENH: Add api for covstruct. Clear __init__. Closes #1917.
ENH: Add api to genmod. Fix cov_struct import.
REF: Explicit imports
REF: Move cov_struct to simpler import.
Merge pull request #1981 from jseabold/cov_struct-import

ENH: Add api for covstruct. Clear __init__. Closes #1917.
Merge pull request #1995 from kshedden/mixedlm_1993

BUG: MixedLM list instead of array closes #1993
ENH: Add plot_predict method to ARIMA models.
DOC: Add plot_predict to release notes.
DOC: Silence bullet list warnings.
EX: Add plot_predict to ARMA example
DOC: Add example for plot_predict
DEV: Misc cruft.
TST: Smoke test for plot_predict
STY: Cleanup
COMPAT: Python 2.6 format args