Add /model/hash REST api point. Returns a hash value of the current (or specified) model in the space.
Update per merge comments.
move hash-tests to space test area for consistently
remove unused constants.
replace the hand build md5 hash -> string function with efficient function call.
merge tjoneslo: Per the Story, add a /model/hash REST API.

https://code.launchpad.net/~tjoneslo/akiban-server/add-rest-model-hash/+merge/155763