Ensure `release_path` returns the correct value

`release_path` will now return the value of `current_path` by default.

Tasks that create a new release (i.e. `deploy`) now explicitly over-ride this
default with a new release path.  This change allows tasks that run in both
deploy and non-deploy contexts to use `release_path` to target the latest
 release when run in isolation, and the new release (before it is `current`)
when run as part of a deploy.
Merge pull request #606 from capistrano/feature/consistent-release-path

Ensure `release_path` returns the correct value