Adding CocoaPods support

This PR adds support to CocoaPods. I've replaced the local third-party dependencies by the ones from CocoaPods main repo. We can work on podspecs for the other dependencies and then remove the last git submodules.

After pulling these commits follow these steps to get it working:

```
$ gem install cocoapods # install pod tool
$ pod install # download dependencies and setup project workspace
$ open MacPass.xcworkspace # from now on use the workspace
```

I've also changed .travis.yml from #261 to include the workspace parameter.