Add optional merging of the session data on set(). 

This prevents data getting lost when multiple asynchronous ajax requests are fired at the server. Each ajax call will do a set() when it's done, but that will overwrite request data that has just been set during the ajax call by another one.

Enable by setting `merge: true` in the options.

This solves problems like this one:
http://stackoverflow.com/questions/5883821/node-js-express-session-problem

Probably needs some tests to be sure it always works as you wish, this pull request is to ask you guys for comments on this approach.