Added permission flags and unit tests.

Added permission flags and unit tests.

New flags:
  --enforce-permissions enables permission checking for GET requests.
  --permission-ttl sets the expiration time on signed locators returned
    by PUT.
  --data-manager-token defines a privileged token for the Data Manager
    to issue DELETE and "GET /index" requests.

PUT now responds with a signed locator if a permission key has been
set.

Unit test TestGetHandler tests the GetBlockHandler both when permission
checking is off, and tests signed, unsigned and expired requests when
permission checking is enabled.

Refs #2328