Use a closure instead of a foreach loop.
Fix handling of untagged strings.
Fix server code:

- "Strict mode" notice about a non-variable being passed by reference.
- Prefix handling in expose() method was messed up.
- Do not expose static methods for objects in expose().
  To expose static methods too, call expose() twice:
  - once with the actual $object
  - then a second time with get_class($object)
Add support for introspection & capabilities.

The new CapableServer class acts as a wrapper around a server,
adding capabilities & introspection support to that server.

For now, "system.getCapabilities" is implemented as a hard-coded list
of currently supported capabilities. We might change this to a more
dynamic mechanism if needs ensues.

Fixes #2.
Fixes #3.
Update examples (both client & server).

- Showcase the introspection features (CapableServer wrapper).
- Add an example of calling an oddly-named procedure (system.listMethods).
- Fix incorrect URL generation in case HTTP_HOST contains a port.
- Add support for passing a request directly to the server using an HTTP
  parameter.
- Showcase \fpoirotte\XRL\Server::expose().