[#42] Remove non-string extras from packages

Remove extras whose values are not strings (e.g. dicts, lists..) from
packages before attempting to create or update the packages on the
target site.

In CKAN 1 it was possible for the values of extras to be other types,
but in CKAN 2 they must be strings, so when harvesting from a CKAN 1 site
into a CKAN 2 site SQLAlchemy would crash when trying to create packages
with non-string extras.

The fix in this commit is to simply remove any non-string extras from
the harvested package. (Alternatively, we could try to convert them to a
string using JSON.)

Fixes #42.