Organizations index should be CclaSignatures json index

Since organizations index actually returns CclaSignatures what is now
the organizations index should be the json format of the
CclaSignaturesController. This also adds the CclaSignature.by_organization
scope so the CclaSignatures returned are unique by organization, this
should resolve any current confusion surrounding this feature.
Merge pull request #620 from opscode/combine-org-qa-fixes

Organizations index should be CclaSignatures json index