Port WebRtc.PeerConnection to freedom-0.6's core.rtcpeerconnection.

This is a very rough draft with regard to code style, but it does
include a version of simple-freedom-chat that uses the new
WebRtc.PeerConnection.  This is working in Chrome, but not yet in
Firefox.
Fixups in response to comments
Rename core-rtc*.d.ts to rtc*.d.ts
Move freedom from a dependency to a peer-dependency.

Node packages that depend on uproxy-lib do need freedom.js, but
their freedom.js may be provided by any one of several different
packages, including freedom, freedom-for-chrome, and freedom-for-firefox.
Since node doesn't have metapackages or disjunctive dependencies, the
best we can do is to specify freedom as a peer dependency (to avoid
version clashes) and a dev dependency (for the samples).
Merge pull request #84 from uProxy/bemasc-pointsix

Port WebRtc.PeerConnection to freedom-0.6's core.rtcpeerconnection.