Add a nice(ish) cmdline interface

This is more or less a sketch of how it will look like, but it's already useful. We can now load `project.pxi` files to gather information about a project. This is handled by the `pixie.project` namespace, which mostly just provides a `defproject` macro.

The `pxi` command is a tiny shell script that dispatches as much as possible to `run.pxi`. So far we need the shell script because we can't execute subcommands yet. Once we support that, we can port everything to pixie.

```
$ pxi help
Usage: pxi <cmd> <options>

Availlable commands:
  deps      List the dependencies and their versions of the current project.
  repl      Start a REPL in the current project.
  help      Display the help
  describe  Describe the current project.
$ pxi describe
hiccup@1.0.5
  Description: A fast library for rendering HTML in Pixie
  Url: http://github.com/heyLu/hiccup.pxi
$ pxi || pxi repl
Pixie 0.1 - Interactive REPL
(linux, gcc)
:exit-repl or Ctrl-D to quit
----------------------------
user =>
```