Added baseline for Assignment 2 code - additional API for program and corresponding implementation as per class notes
implemented pushVariable
modified performOperation to accept a dictionary of variableValues
changed allowableVariables to an NSSet
modified runProgram to accept dictionary of variableValues
added placeholder for variableUsedInProgram
added isOperation to API and implemented using NSSet operationSet
removed allowableVariables
implemented variablesUsedInProgram
expressed variablesUsedInProgram NSSet block test more explicitly just for testing
added old performOperation methods to support existing controller implementation in the case where no dictionary passed to brain
removed static OperationSet and instead put it inside isOperation
implemented runProgram using variables
updated variablesUsedInProgram to return nil if no variables are in the program
added pushOperation method to assist unit testing