Port to iOS

:warning: **NOT READY FOR MERGE** :warning: 

Initial port to iOS. This port focuses on FTP and SFTP compatibility.

We might consider using [curl-ios-build-scripts](https://github.com/brunodecarvalho/curl-ios-build-scripts) instead of having curl and the very draft [libcurl-for-iOS](https://github.com/EnthusiasticCode/libcurl-for-iOS).

Also I might start using [github/objective-c-conventions](https://github.com/github/objective-c-conventions) while retouching the code. What do you say?