natural-sort entity records in CSV export
remove file writing option from models.metadata_files

Just look for files. If some other func wants to cache the results
that's cool but don't make things more complicated than they have
to be.
remove unused functions
nose and docstring tests for many batch functions
functions for converting between object data and CSV
replace print() statements with logging
CSV load/dump functions use '' instead of None for null values

Prevents occurance of null field valuess being converted to
the string 'None'.
validate_row runs csvload_* on value before validating it

After csvload_* the value is converted from its CSV form to
its Python data form.
update_entitys/files now stage modified files

Modified files are staged to the index but not committed.
more logging stuff
validate fields in header list

Iterate on the list of fields actually in the document
(i.e. the header) instead of the valid_values dict
make the test_entities function actually load entities
use correct base path when loading files