Server modes

This pull request accompanies xp-framework/xp-runners#18 and implements the `xp.scriptlet.Server` class, an HTTP server written in pure PHP.

```sh
# Start the development webserver, which reloads the application every time
# just as you're used to from PHP.
$ xpws -r doc_root

# Start the PHP webserver, which keeps the application running across requests
# just as you're used to from e.g. NodeJS
$ xpws -m serve -r doc_root
$ xpws -m fork -r doc_root
$ xpws -m prefork -r doc_root
```