Add multiple NumPoints selection methods

This builds upon the work from @russelhaering here: https://github.com/rackerlabs/blueflood/pull/167 -- additional context available there.

After this PR:
 * There are now configuration options for:
    * Configuring interval we receive data at, this will only affect the way we do numPoints calculations.
    * Which numPoints selection algorithm to use.
 * There are now 3 selection algorithms
    * Linear distance (status quo)
    * Geometric distance added (new default)
    * Less-than-equal (Less than or equal to requested number of points.)