Add time parsing that supports absolute and relative time

[carrieedwards]

This PR expands CarbonAPI's handling of time values by parsing both absolute and relative time. This is needed to meet the requirements from the Graphite web documentation as well as to match Graphite web's functionality (see here for the implementation).

This will fix time handling for several functions, include the linearRegression, timeShift, and verticalLine functions.

[Civil]

Hey, JFIY: I'm not looking at this PR because it's marked as a Draft.