Provide a way to set the full path to the socket.io instance

Hi! 

Very nice project.
On the server I am working on, there are two different socket.io instances: one which is located at /socket.io and another one which is located at /path/to/socket.io. Currently, the client easyrtc api doesn't provide any way to specify that we want to use the second one. This pull request fixes it :) The method setSocketIoPath gives the possibility to set /path/to/socket.io as the full path to socket.io and then the resource attribute is passed (if defined) in the config options of io.connect.

I didn't find any development best practices so let me know if there are some stuff you would like to see changed.
Cheers!