Merge pull request #637 from guyrt/pyflakes-cleanup

Pyflakes cleanup
BF: Make RLM stick to Huber's description.
Merge branch 'master' of https://github.com/statsmodels/statsmodels
"Fixed bug in statsmodels/regression/linear_model.py"
" Fixed the 1-d error checking"
Merge pull request #649 from Daniel-B-Smith/master

Bug Fix, input validation sigma
Merge pull request #648 from VirgileFritsch/master

BF: Make RLM stick to Huber's description
DOC: intro to formulas
Merge pull request #619 from vincentarelbundock/formula_example

DOC: intro to formulas
ENH: fixing pyflakes / pep8, trying to improve readability
ENH: addressing discussion
ENH: consistency for quation marks
ENH: missing commit
ENH: ... a few more missing quotation marks.
FIX: adding back directives
ENH: notes on examples
ENH: misc
- adding directives where missing
- fixing categorial interaction plots example
- improving example.rst
Merge pull request #652 from dengemann/fix_pep8_doc

WIP: fixing pyflakes / pep8  closes #644 closes #645
DOC: fix example_interactions.py (ipython directive reports indentation error with failure try except)
a typo in the doc strings of mcnemar in runs.py correction parameter continuity corection -> continuity correction