Add variable interpolation to notification messages.

This includes required refactorings to enable replacing the http client (for
testing) and moving the NotificationReq type definitions to the "notifications"
package, so that this package doesn't need to depend on "rules" anymore and
that it can instead use a representation of the required data which only
includes the necessary fields.
Merge pull request #355 from prometheus/feature/interpolation

Add variable interpolation to notification messages.