Persist ints and force input type to integer in NumberRangePreference.

Since NumberRangePreference is only going to be used for ints, it
makes more sense to persist and retrieve values as ints only. Also,
don't make the input type configurable; it's always integers.
Merge pull request #41 from hssm/num-range-values

Persist ints and force input type to integer in NumberRangePreference.