clean up missing mapping code

This commit cleans up code that fails when
mapping keys do not exist in the list of
expected classes.
Change hiera->data_mapper override order

This commit changes behavior:

previously, data_mappings would override
the same hiera key if it existed.

This behavior was mainly due to performance implications
of implementing this in hiera.

Now that we are pre-compiling, I can have individual hiera
values override the data_mappings.
Unit test updates
Merge pull request #13 from bodepd/minor_dev

Minor dev