Updated plugins version to support sbt 0.13
Switched back to sbt 0.12.4
Added expression parser
Updated ExpressionParser grammar
Repaired ExpressionParser
Added access rights for ExpressionParser
Added Calculator plugin
ExpressionParser tests
Fixed tests bugs
Created new instance of ParseRunner for every parsing run
Added CalculatorPluginSpecs
Added more Calculator plugin tests
Removed unnecessary string parameter in process method
Merge pull request #7 from DrunkenPandaFans/calculator

Calculator :shipit: