discovery assitant and some CalDav.js improvements

owncloud is not yet fully working... still figuring out how discovery should be done here. Also fiddled a bit with Yahoo and planning to try google calendar also. 

The biggest change is that oncreateassitant do not include discovery anymore. That is moved to another seperate assistant and service call. It is now done during sync, if necessary (i.e. if calendar or contact home folder is unknown). This is much better, because previously, if discovery failed, the account was in some undefined state and the URL was not saved in the DB => sync always failed, one had to delete the account and try again. 

The discoveryAssistant can be called from command line and can even be abused to change the URL after account creation... this makes debugging / evolving the discovery much easier! :-)