XPWS and persistent PHP webservers

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
```

To address static resources without checking with `is_file()`, you can supply a section in the `web.ini` file:

```ini
[static]
resources=".css$:{DOCUMENT_ROOT}|^/image/:{DOCUMENT_ROOT}"

[app]
mappings=...
```