Handle include_* arguments in compare_metadata()

include_object, include_symbol and include_schemas are very useful,
when you need to specify what objects you want to be compared.
Modify compare_metadata() public API function, so that it takes those
arguments into account and pass them to _produce_net_changes().
Merge pull request #9 from malor/compare_metadata_include_object

Handle include_* arguments in compare_metadata()