BUG: When increasing the length dynamically, the rolling panel was
getting filled with the wrong datetimes and causing errors.

Updates the logic for addressing missing datetimes and adds unit tests
for the 2 main cases (no missing datetimes, and some missing datetimes).
BUG: rolling panel data became misaligned after extend_back
BUG: Fix ta-lib url in vagrant init shell script.
ENH: Replaces the simple transforms with history calls. Switches
transforms to quantopian syntax.

Adds the sid attribute to the siddata so it is aware of which security
it represents.
MAINT: Makes the date_gen skip non-market times.
MAINT: Updates the add_trading_days to use the index of the date for a
more efficient means of jumping larger gaps of dates.

Adds a docstring to explain the usage of the function.
DOC: Updates the comments around the caching in siddata
BUG: Fixes a bug in the test date_gen for non-daily timedeltas
DOC: Adds the new transform work to the release notes.