XPI - Install XP Modules

This pull request adds the ability to install modules released with the new build system and/or hosted on GitHub.

Preparing
--
Before being able to use this, add a directory to where to install the modules to.

```sh
$ cd ~/devel/xp
$ mkdir modules
```

...and add then add that to your `use_xp` setting inside *xp.ini*:

```ini
use_xp=~/devel/xp/5.9.0RC8;~/devel/xp/modules
```
(On Un*x, use ":" to separate the paths!)

Basic usage
--
Once you've prepared this, you can start managing your modules:

```sh
# This will install the newest version of the module "module" from the vendor "vendor"
$ xpi add vendor/module

# This will install a specific version which must've previously been released
$ xpi add vendor/module 1.0.0

# This will remove the module
$ xpi remove vendor/module

# This will list installed modules
$ xpi list
```