Change SocksToRtc to have a freedom-compatible interface.

This change also adds a manifest.json for SocksToRtc, but this
manifest is not yet used anywhere or verified by the build.
Technically, the correct type here is something like "WebRtc.SignallingMessage OR Churn.ChurnSignallingMessage", because either kind may be passed, depending on the value of the parameter `obfuscated` that is passed to `start()`.  Previously, ChurnSignallingMessage was a subtype of SignallingMessage, but as of https://github.com/uProxy/uproxy-networking/commit/069602034212daff969f5028544876831ee44df0 that is no longer true, and the two interfaces have no non-trivial shared supertype.

The good news is that these sorts of [union types supposedly will be supported](http://blogs.msdn.com/b/typescript/archive/2014/11/18/what-s-new-in-the-typescript-type-system.aspx) in Typescript 1.4.