Make commands extendable (updateable)

Hello Camme,

Another project requirement (same project summarised in [#57](https://github.com/Camme/webdriverjs/pull/57)) highlighted a need to be able add custom commands and also edit existing ones.

This patch makes it possible to update existing commands by doing the following:

client = webdriverjs.remote({
                     customCommands: {
                            end: function(){} //exisiting
                            quit: function(){} //new command (can also use client.addCommand.quit)
                     }
	       });

Regards,

Clara