Support parsing async functions and await expressions

This is essentially the same as https://github.com/ariya/esprima/pull/234, but I'm not sure when/if that PR will be merged.

Since `async`/`await` is an ES7 feature, it seems to fit with other distant-future and/or custom syntaxes (e.g. JSX) that have been implemented in `esprima-fb`.

If we merge this PR, then I can go ahead and merge https://github.com/facebook/regenerator/pull/101 without worrying about depending on my own fork of Esprima.

cc @jeffmo @zpao @lukehoban @arv @wycats
`async` is supposed to be a contextual keyword like `yield`. `async[NoLineTerminatorHere]function`