Create a Caller library gem

Libraries are calling caller and then regexp parsing a part caller frame lines to get file, line, method, etc.. out. We could make a 'require "caller"' which allows Caller.caller (specifics not so important) which instead of returning an array of strings it will return an array of Caller instances. In JRuby, we can just create these directly in our impl and if we put this out as a gem then we can do the regexp parsing on MRI side. Then libraries like rspec can depend on 'caller' gem and things will work great everywhere and not be stuck with wacky caller regexp parsing in n-libraries.