Improve build tags for Wasm

Fixes #515

This includes a few small and closely related changes:

1. All occurrences of the build tag `+build js` have been changed to the
   more precise `+build js,wasm`. This will exclude the files from being
   included by third-party compilers like GopherJS, with which they are
   incompatible.
2. Some files which are incompatible with JavaScript/Wasm now have the
   correct build tag (`+build -js`) so they will be excluded from Wasm
   builds.
3. Some configuration options which are incompatible with
   JavaScript/Wasm (or at least the current bindings) will now no longer
   appear in Wasm builds. This meant creating new files with new struct
   definitions and the appropriate build tags.

settingengine_test.go

@@ -1,3 +1,5 @@
+// +build !js
+
 package webrtc
 
 import (