Adds missing BSONJavaScriptIdentity

I have added BSONJavaScriptIdentity as it was missing and creating BSONDocument with BSONJavaScipt was impossible (compilation error). Because of this, I could not run a raw command with JS - mapReduce in this case. 

The code that caused the compilation error (but works now): https://gist.github.com/wjurczyk/7578020