Add support for processor param in contexts endpoint

Fix #228 .

This is done via the context form. The query processor and tree are now
passed to the form and those are used when setting the count on the
instance being saved in the query form when the count needs updating.

This behavior will need to be update for the 2.4(master) branch as much of this code has changed both in Avocado(count fields were deleted) and in Serrano itself(stats resource added) but this works on the 2.3.x branch and will bring this functionality to the users requesting it as they still use the stable releases.

Signed-off-by: Don Naegely <naegelyd@gmail.com>