Implement loading a single plugin archive

This pull request includes the extension finder reset enhancement.  The plugin manager is now able to load and unload a single plugin properly (tested with Gitblit).

This is a simplified implementation which does not account for resolving dependencies.  I tried using your existing resolve method, but that actually discards previous state.

It's a good starting point for simple plugins, but will no doubt need refinement.

I'm noting that your Eclipse is not configured to trim trailing whitespace which is partly why my diff looks large.  Looks like there is a line-ending difference too.  Sorry about that.