Extract hook signature logic into HookSigner

The `PluginHookLoader` was getting complicated since it combined
signature calculation with hook loading.

Extract the signature calculation and verification into a separate
helper class.

Change-Id: I7632eecb6a454846126bac146fd94c578b57e7c9
Reviewed-on: http://gerrit.causes.com/37385
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>