Improvements to demo usability

My first time using the PeerJS demo left me somewhat confused, so I took a closer look and made some small improvements to its usability.

* Listen to `submit` event for connection and message send. This mirrors the behavior of chat clients that most people are already acquainted with.
* Increate the font size of the footer. These warnings are really important, and I actually passed over them the first time I used the demo because they were so small.
* Use an `h2` for "Now chatting with X" message. This makes it obvious your connection was successful.
* Modified the markup for messages. Bold user IDs and small margins help differentiate messages.
* Use sans-serif font

Cheers! :beers: