fixed bug with multiple assignment

if you get() multiple times you run assign() multiple times. This is
not the desired behavior. This patch fixes this, and also includes a
unit test to make sure this doesn’t happen.
updated unit tests to reflect change in salt func

The salt function changed in 0.2.3, which re-shuffles assignments. This
fixes the unit tests to reflect that change.
Initial version of PlanOut for Ruby

Super pre-alpha version of PlanOut for Ruby. Includes Assignment
object, UniformChoice, WeightedChoice, RandomInteger, RandomFloat, and
BernoulliTrial.
Merge pull request #18 from facebook/multiple_assignment_fix

Multiple assignment fix