Added support for verify, doc_upload and remove_pkg actions

I'm experimenting with using `pypiserver` for my testing of distlib, and the standalone option is very handy. I added support for actions which I needed in my testing:

* `verify` (does nothing, but returns success)
* `doc_upload` (checks that a valid zip file is uploaded, and that it has an index.html)
* `remove-pkg` (needed since tests run repeatedly, to avoid 409 errors on file_upload actions).

You may find these useful, so I thought I'd share. I haven't added tests, as I'm not that familiar with `py.test`, but I noticed that there don't seem to be tests for POST operations anyway (e.g. `file_upload`).