This PR allows a template to specify the Vault grace duration.

Fixes https://github.com/hashicorp/nomad/issues/2922

This PR makes the example JSON job submitable via the HTTP API.

This PR updates the api documentation to reflect 0.6 HTTP API and the
JSON job specification as well.

Caught some typos.  Made units separate from the numbers 1GHz -> 1 GHz
after talking to Nick about questions of style (this has the side effect of making future spell checking easier).

Fixes https://github.com/hashicorp/nomad/issues/2530

Closes #2237

These are things I noticed as I read through the documentation.
There are a couple of minor substantive corrections as well.

For easier lookup.

* while HCL job-spec uses `operator`, JSON uses `Operand`
* `Operand` is required, it no longer defaults to `==`
  (@see nomad/structs/structs.go#L2297)