THRIFT-2576 Implement Thrift.Protocol.prototype.skip method

Thrift.Protocol.prototype.skip was not implemented yet. 
Due to the exception that the previous implementation throws, Thrift client in JavaScript has issues on backward compatibility.

A quick patch would be setting skip() to an empty function. Setting skip() to an empty function resolves this issue because skip() is always followed by readFieldEnd() in all generated codes. 

However, I implemented skip() correctly so that it can be used in different contexts.