Frame little/big endian overhauling in frames

[FrancescAlted]

Frame serialization needs an overhauling so as to make sure that:

1. All ints for msgpack are to be stored in big-endian (aka network-endian)

2. The rest of ints (e.g. offsets of chunks in frames) are to be stored in little-endian

So yeah, we have a nice mix of big and little endian embedded in the frame serialization format. The reason for this apparent schizophrenia is that msgpack is strict about using big-endian, and Blosc has historically used little-endian for serializing the ints in the chunk header (which makes sense provided the prevalence of little-endian CPUs nowadays).

We will setup some macros that should make this big/little endian handling easier.

[aleixalcacer]

the macros wil be introduced in #247