Add support for DELETE method in v1/data API

[tsandall]

The OPA v1/data API supports deletion/removal of data via JSON Patch, however, in some cases, it's easier to call v1/data with DELETE instead of crafting the JSON Patch. For example:

Instead of:

PATCH /v1/data/foo

[
  {"op": "remove", "path": "bar/baz"}
]

Callers would:

DELETE /v1/data/foo/bar/baz

The implementation should be relatively straightforward:

• Add a new v1DataDelete function to handle DELETE calls
  • Open a write txn
  • Call storage.Store#Write(remove, <path>, nil)
  • Commit txn on success (or Abort on error)
• Register handler with the others
• Update rest-api.md with docs on DELETE support (copying format from other sections)

[BenderScript]

I will take care of this one. I have interest on having it working (;-)