Resolve issues #149 and #155.

Move 'tuf.client.updater.Updater.refresh()' call in interposition to its updater's __init__().  The refresh() was previously executed on every target request.

Add tuf.interposition.refresh() for integrations that may require a manual refresh of top-level metadata (rare).
Merge branch 'repository-tools' of https://github.com/theupdateframework/tuf into repository-tools