Added permission helper functions.

GeneratePerms returns a string representing the signed permission hint
for the blob identified by blob_hash, api_token and timestamp.

SignLocator takes a blob_locator, an api_token and a timestamp, and
returns a signed locator string.

VerifySignature returns true if the signature on the signed_locator can
be verified using the given api_token.

Refs #2328.
Update docs. (refs #2328)
Incorporating code review. (refs #2328)
Resolve code review (closes #2328)

Fix doc comments and argument names for the expiry timestamp arguments
for SignLocator and makePermSignature.
Merge branch '2328-keep-permission-hints'