Expand time parsing to handle both absolute and relative time

[carrieedwards]

Is your feature request related to a problem? Please describe.
According to the Graphite web documentation, time windows passed into some functions, such as linearRegression, timeShift, and verticalLine, can be of a variety of formats, in either absolute or relative time. Some examples of formats that should be parsed correctly are:

"20091231"
"noon+yesterday"
"january+1"
"04:00_20110501"
"monday"

Currently, only interval strings (such as "1m", "3h", 7d, etc) are parsed.

Describe the solution you'd like
The time formats supported by Graphite web should be properly parsed and handled.

Additional context
Graphite web's time parsing functions can be found here.