Add images & image repositories

Add initial support for images & image repositories.

Add registry webhook to support adding images and tags
to image repositories when a registry's image repository
is updated.
Merge pull request #45 from ncdc/images

Merged by openshift-bot
Disable coverage for Travis + go tip
Merge pull request #64 from ncdc/disable-coverage-travis-go-tip

Disable coverage for Travis + go tip
WIP Card#287 - Allow git webhooks to trigger build on git push
WIP Card#287 - changed the webhook mgmt, added git webhook implementation
WIP Card#287 - Changed plugin registration mechanism, now plugin is registered
manually inside cmd/master/master.go along side with installing the REST API.
Changed GetBuildFromPlugin method name and other house cleaning.
WIP Card#287 - Changed controller registration, moved gitwebhook to github package,
moved test files to gitub/fixtures dir, removed unnecessary elements from github
types.go, renamed types.go -> types_v3.go.
Rebased and added build integration, extended tests as well.