Add drain method to WebSocket{,Context}

[jean-airoldie]

This allows the user to retrieve unsent queued messages
after a connection died.

Fixes #94

[daniel-abramov]

Just a couple questions:

1. Is it still a WIP (work in progress) PR or do you plan to extend it?
2. Would it be possible to split the PR into multiple commits, so that the changes which only change the formatting of the code can be kept in a separate commit?

[jean-airoldie]

1. Nah its not WIP anymore, forgot to rename the title.

2. Sure thing, didn't realized that it picked up my rustfmt. I'll make a separate commit. I'll also add the travis logic to check for the formatting. Turns out that its not actually due to code formatting, but the git diff kinda messing up. I extract part of a function into another function and the indentation makes it seem like its a formatting thing.

[jean-airoldie]

Also I'm not sure whether agalakhov's comment #94 (comment) was addressed.

[jean-airoldie]

Closing in favor of #104.