JIT ("Just in time") compilation [in-memory]

This pull request implements xp-framework/rfc#271 adds a specialized class loader for the efficient "edit / save / run" paradigm at development time. 

```sh
$ cat > HelloWorld.xp
public class HelloWorld {
  public static void main(string[] $args) {
    util.cmd.Console::writeLine('Hello ', $args.length ? $args[0] : 'World');
  }
}

$ xp HelloWorld
Hello World
```

This will compile the source files' code, define and load the class and finally execute it. No files are written to the file system as part of this process. An alternative pull request which does this is #34.

### Installation
The `xp.compiler.JitClassLoader` gets registered by adding `/path/to/compiler/jit` to the `use` line instead of just `/path/to/compiler`.

### How it works
We need to use a small trick to register the class loader: Using the `/path/to/compiler/jit` path adds `jit/init.php` to the class path, which then hooks itself into the bootstrapping process and as soon as the ClassLoader class has been loaded, it registers the `JitClassLoader` instance as delegate of it. *Note: This is what modules where designed for - as long as we don't have them, we need to use the somewhat magic method described here*