ENH: add ability to get daily multi-period forward returns rather tha…

…n only cumulative

Merge pull request quantopian#328 from luca-s/new_pandas

MAINT: code refactoring to make it compatible with pandas 0.23.4

Merge pull request quantopian#320 from quantopian/little-s-fix-1

DOC: Small typo in docstring

TEST: added tests for perf.mean_return_by_quantile (quantopian#318)

BUG: compute_mean_return_spread returns error if no std_err argument

Issue quantopian#299

TST: added test cases for holidays detection

DOC: example notebooks use deprecated yahoo API (quantopian#285)

The NBs now uses google API

Revert "Removed group_adjust/group_neutral flag from information anal…

…ysis API"

This reverts commit 2eed755.

The original commit was a mistake as the IC is computed both on the full
factor and by sector, so the flags don't change the IC at sector level but
they are still needed when computed the IC for the full factor

Updated example NBs with new API and improved price DataFrame explana…

…tion