Added SinglyConnection, a simple wrapper for NSURLConnection.

The purpose of SinglyConnection is to normalize the error handling and
JSON parsing functionality that is duplicated throughout the codebase.
This class is currently private until I am happy with the API, at which
point it will become a public API.
Don't generate documentation for SinglyFriend or SinglyConnection.
Migrated SinglySession to use SinglyConnection for requests.
Migrated SinglyService to use SinglyConnection for requests.
Migrated SinglyLoginPickerViewController to use SinglyConnection for requests.
Merge branch 'master' of github.com:Singly/iOS-SDK