Fix commit hooks and symbolic properties as binaries

First problem solved: 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.

Second problem solved: some properties (quorums, repl) have symbolic values represented as atoms in the encoding but can be set over HTTP and stored as binaries. This coerces them appropriately.