feat(chat): Add /help slash command.

Lists current available commands with client-side system messages.

resolves #5
fix(chat): Resolve improper use of $apply with $timeout.

Previously I thought I had to call $apply prior to calling scrollToBottom because the digest hadn't finished. A simple 'next tick' delay of scrollToBottom is enough, and doesn't have issues of repeat calls to $apply.

resolves #3
feat(chat): Limit client side history to 500 lines.

resolves #4