[#224] Better feedback when updating IATI XML projects

Add the Akvo pk to the output from api/cordaid_importer.py when updating
an existing project
[#224] Fix missing benchmarks bug

Some benchmarks are not created because of identical Benchmark names in
several categories and code relying on the uniqeness of same.

Fix by using the business_unit_categories lookup dict to assign the
correct category to a benchmark.