Improve comments; independent client/server pings

This commit fixes up most (or all) existing comments throughout the
code base. Comments should now accurately reflect the behavior of the
code.

Additionally, the behavior of ping/pong frames has been changed:

Rather than simply replying to client-sent ping frames with
corresponding pong frames, a server-side Session will now send pong
frames periodically (irrespective of whether a ping frame has been
received from a client).

TODO:
* Tests
* Documentation (README)
* Profiling
* Builtin Lua Scripts