Added serializer to interpreter

The serializer has partial specializations for all templates. It is a
good basis if the connector/database requires strings close to the
standard.

The interpreter is unspecialized (and uses a static assert to say so).
It is a good basis if the connector/database requires a different
interpretation, e.g. re-writing the expression tree.

The interpretable_t implements three methods for interpretation:
serializing with the sqlpp::serializer_context, serializing with the
database's serialization context and interpretation with the database's
interpretation context.