Merge remote-tracking branch 'upstream/master'
Add default constructors to abstract classes
Assert
Merge pull request #165 from CodaFi/default-constructor

Assert when calling -[MCOAbstractMessage init] or -[MCOAbstractPart init].
They are the wrong designated initializer.
Fixed regression introduced in de51dd02d4c16d5c6bc89a2fe0e49ffda79f2a33