Pull Request!

Hi Chakrit

**Don't merge yet!** I think we need to fix issues below first. Need your help ;)

I have merged your master branch changes to mine, fixed conflicts.
I can run example target and can connect to sip server.

Additions
- add GSCallInfo class
- change `-connect` method to `-connectWithCompletion:`
- remove unnecessary KVO codes (`-willChangeValueForKey`, `-didChangeValueForKey` stuffs)

There are some issues I don't know how to solve. listed below.
major problems
- cannot build for iPhone retina (4-inch 64-bit). I see the fat lib already has arm64. But it still doesn't work. May be there's some bad config in project.
- `docs` submodule cannot be downloaded (I removed it in order to make it work, may need to put it back).
- after `pjsip` submodule is downloaded, I still need to copy 'pjsip/config_site.h` to `pjsip/src/pjlib/include/pj/config_site.h` to make it build.

minor problems
- some of your sip account doesn't work. please check. (one is `chakrit@getonsip.com`)