Update subversion module to work better with check mode

Adds needs_update() method which will inspect the checkout for the
current revision and then the HEAD.  If the local check out is behind
HEAD, this will report changed=True and supply the revision numbers.

Example output from check mode:

    host | success >> {
        "after": "Revision: 19021"
        "before": "Revision: 19006"
        "changed": true
    }