retain is bit 0 of the flags, not bit 3 (aciidgh#24)

- retain is bit 0 of the flags, not bit 3
- Add publish message packet header test

Swift 3.0

Merge pull request aciidgh#5 from adolfo/feature/add-ssl-support

Add SSL support to MQTTSession

Merge pull request aciidgh#1 from adolfo/feature/add-podspec

Add CocoaPods Podspec