Keep alive support

[boenrobot]

Added the ability to set up a keep alive interval that will close the connection if it does not receive a reply in time.

I've modeled the code largely after the one found in the ratchet server, except it's only monitoring the one connection the client is connected to, and the timer is returned, so that it can potentially be canceled later (or be replaced).

Help with adding tests will be appreciated, as I can't find any test that exchange messages over a real connection - just mock connections or mock connection creation.

EDIT: Commit updated to use my github email... Because I committed this from work, it originally used my work email. Oops.

[TheLevti]

Would be amazing to get this support out. I suspect I have such a case where sometimes the connection is just gone and I am stuck listening forever.