cordova plugin add https://github.com/joseph-onsip/phonertc

I can't figure out how to make the plugin a submodule and have it still work if
it's updated, so here's a link to the latest commit at time of clone:

https://github.com/joseph-onsip/phonertc/commit/e7c7f7974d253cd2b30712add4e3dc682c647ee2
git submodule add https://github.com/joseph-onsip/SIP.js.git sip.js
cordova build && cordova run

- `git add` files that changed
- .gitignore new files
.gitignore platforms/android/assets

git rm platforms/android/assets
git add Makefile
.gitignore www/js/sip.js
www/index.html: load www/js/sip.js