Fixing issue #79

Added a set of trait classes that can be used to get an input archive
from an output archive.  Requires specializing a struct for each direction or
alternatively using the new macro CEREAL_SETUP_ARCHIVE_TRAITS(InArchive, OutArchive).
This has already been added for all built in archive types.  This is currently only
used for minimal serialization.

load_minimal type traits now correctly use the output archive to check the existence of
a corresponding save_minimal and get its return type, using the new get_input_from_output
type class.

Added a test for this case into the minimal structs test.

Sandbox_vs needed the new macro to become compliant.