Major rewrite: "Metav as a release orchestrator"

[jgrodziski]

metav as an orchestrator with a pipeline of well-defined steps (with a protocol see below), see chain of responsibility.

• rollback on every steps of the release process (with an up/execute and down/rollback function for each steps of the release process, see the protocol below)
• hook on every steps of the release process with pre and post hook
• dry-run option (plan)
• push at the end of the release "transaction": release -> package -> push
• optional: commit that add the signature (sha1) of every package built during the package stage

Metav as very easy way to release/package/push a library, just drop the dependency and everything is taken care of.

Each step should be see as a "plugin": like slack notifications, mail, build jar, etc.

Useful step "plugin":

• slack notif
• changelog templating and building
• jar, uberjar

(defprotocol Step
   (plan []) 
   (execute []) 
   (rollback []))