restapi: Add HexUtils

This patch adds a utility class used to convert hexadecimal identifiers
to strings and the other way around. It will be used by resources that
don't have a identifier, so the identifier will be generated as the hex
encoding of the name.

Change-Id: I703763e11d26f2a1a0c6b0984249acc8a6744f95
Related-To: https://bugzilla.redhat.com/1132259
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>