preliminary objective-C implementation of a sereal decoder

Decoding of buffers created from the perl encoder seems to work (but needs more testing).
Roundtrip is not guaranteed at the moment because there is no perl-compatibility mode implemented yet.

An encoder and an higher level API will come once the decoder will be fully implemented
(together with tests and a perl-compatibility mode to allow encoding back objects and other datastructures
exactly the same way they originally came from a perl encoder)

Note: this is an early push ... only decoding has been implmented,  work is in progress and no high-level API has been really defined yet