REF: Remove sign_test from sandbox
REF: Add sign_test to stats.descriptivestats with cleanup
REF: Make import and test safe
ENH: Add safe nottest decorator
ENH: Decorate sign_test as not a nosetest
TST: Test sign_test.
STY: Pep-8 and comment cleanup
DOC: Add sign_test to nonparametric test section
Merge pull request #761 from jseabold/fix-642

ENH: Fixes and tests sign_test. Closes #642.
ENH: Adjust for k_trend in information criteria. Closes #324.
ENH: Add df_model to AR model