call back if the package has been downloaded
default after_download proc to do nothing
add integration test for after_download callback
nexus classifier bug

the original code assumed whatever value after
the extension is the version.  This is not
necessary true since the classifier can come
between the version and extension.
integrated after_download to deploy resource

- included integration tests
wire after_download hook to deploy resource
include nexus package test
Merge remote-tracking branch 'celkins/feature/support-maven-classifier' into after_download

    Conflicts:
        providers/deploy.rb
include nexus_package as default integration test
include after_download in readme
Merge pull request #107 from aaronfeng/after_download

After download callback for artifact_package