Add support for @GrabResolver to AetherGrapeEngine

@GrabResolver can now be used to add a repository to the list that is
used for dependency resolution. Any repository that is added via the
annotation will then be available for the lifetime of the
AetherGrapeEngine instance. In reality, this equates to the lifetime
of the Boot application. This is in keeping with the documented default
behaviour [1]: "By default, the grape subsystem is shared globally, so
added resolvers will become available for any subsequent grab calls".

[1] - http://groovy.codehaus.org/api/groovy/lang/GrabResolver.html

[bs-345]
[60145036]