webrtc-adapter dependency changed to npm dependency from git submodule

[NTARelix]

AdapterJS depends on webrtc/adapter (webrtc-adapter on npm). We currently declare this dependency via git submodules when ideally it would be in package.json. There are multiple reasons for this (one of which I've run into with bug #262, as well as being unable to install this dependency in a minimal CI slave that doesn't have git).

The build involves hooking into the npm lifecycle (see postinstall in package.json) to build this webrtc-adapter dependency before injecting it into this package's output. I've removed the submodule in favor of a pre-built npm dependency.

[johache]

I think that this is very good idea.
We never merge anything straight into master though. Could you change your PR so that it goes into dev instead ?

Thanks,
J-O H