Add another flag for the presence of each precommit and postcommit
properties. Fixes #32.

I wouldn't normally change a whole bunch of field numbers, but this has
not been finalized into a release. This resolves a problem found in
the bucket_props_roundtrip riak_test.
When some props are set via HTTP, they are binaries, not atoms.
Merge pull request #33 from basho/sdc-fix-commit-hooks

Fix commit hooks and symbolic properties as binaries