Remarks w.r.t. output of `--help`

* `--prefix` doesn't mention anything about also adjusting prefix of default repository path set, it should?
* for options that have a string as default value, quotes should be showed around the default value (e.g., `--buildpath`, `--logformat`, `--prefix`)
* the default value for `--logdir` doesn't make sense, since it's the tmp dir received from `tempfile.mkdtemp`; it changes every time you run `--help`, and will thus also change when `eb` is used
* `SvnRepository` is not shown as a valid choice for `--repository`, but it does show up with `--avail-repositories`, how does that happen?

I guess the pull request is ready for merging once these remarks are resolved.