Addon signing service (bugs 1070189, 1070191 and 1070227)

Sign addon files during review process (bug 1070189)
Management command to sign addons (bug 1070191)
Signing all addons script (bug 1070227)
Merge pull request #353 from olivier-m/1070189-trunion-api

Signing Service initial support (bugs 1070189, 1070191, 1070227)