Adding ProtoCall serialization library

Add serializer and deserializer classes for
Avogadro::Core::Molecule to allow this data type
to be included in ProtoCall messages. The serialization
is performed using a simple length-prefix approach and
utilised protobufs streaming classes.

Change-Id: I501db702d0e4a3e193e4850d027695ce0fcf96a5
Merge topic 'protocall-serialization' into master

389b4b69 Adding ProtoCall serialization library