Add GET parameter for specifying the query processor

Fix #188.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
This happens because [this line](https://github.com/cbmi/serrano/blob/master/serrano/resources/field/values.py#L71) in Serrano calls the `random` method on the `DataField` object which in turn calls `random.sample` on [this line](https://github.com/cbmi/avocado/blob/master/avocado/models.py#L667). `random.sample` throws the `ValueError` if the number of random items being requested is larger than the population being drawn from. I will update Serrano to account for this and update this PR.