Update SPEC with some feedback from Ben Johnson.
Add field in Response indicating presence of key in store.
Always return a Response; include KeyOK field.

Previously, if there was any error, the backend could respond with with
a nil Response, leading to panics.
Multiple HTTP server improvements.

* Use proper HTTP response codes; i.e. 201 for key creation, 404
  for keys not present in the store.
Add version information.
Set KeyOK in Get.
Add HEAD support.

HEAD requests are now supported to enable checking of the version
header. This required some shuffling around of code, but nothing
major.
Add version information in the common package.
Add basic version of a client package.