Use plain 'for' loops instead of 'for in' loops on arrays 

Addresses issue #82.  This is pretty straightforward, I think, but I did have two comments:

* `RTCSession.earlyDialogs` was defined as an array but treated as an object (indexed by callid+fromtag+totag).  I changed its initialization to match.
* `RTCMediaHandler.init` used `this.session.ua.configuration` four times in two loops, so I cached it as a local variable.