Implement serialize for sequence

Implement serialize for sequence
This manages to save Sequence, motif, and annot using boost::serialization.

I added a serialize_simple to test_sequence.cpp

On the down side with this patch mussa now depends on boost::serialize
libraries.

(And it was harder to get working than I'd expected. I missed the fact
that there seems to be limits about sending non-const objects to an
archive class. I also found it useful to put the
template<class Archive> serialize() function into the header with
and to mark the class with BOOST_CLASS_EXPORT(). If one includes
the boost/archive/*archive classes before including sequence.hpp,
the BOOST_CLASS_EXPORT will force the appropriate template to be
instantiated.)