Added an "asString" function in mongoUtil that can set a value to be str...

Added an "asString" function in mongoUtil that can set a value to be string in mongodb


This is in reference to the issue talked about in https://groups.google.com/forum/#!topic/cfmongodb/Eeia92jt7Uo

Instead of using StringBuffer, I created a separate class for it to avoid breaking any existing implementations.