Rename join() to something that reflects its intended use better

[avayvod]

The suggestions comes from the discussions with Mozilla on the mailing list: https://lists.w3.org/Archives/Public/public-secondscreen/2015Aug/0017.html

I think connect() was what Mark F and both Shih-Shiang & Jonas agreed about.

[markafoltz]

Re-reading the use cases from the mailing list thread, I would now lean towards reconnect() as suggested by Jonas: they are all examples of a page reconnecting to an existing presentation.

Jonas and Shih-Shiang, what do you think?

[schien]

reconnect() and resume() are both ok to me for session resumption.

[sicking]

reconnect() sounds great to me. Thanks!