Remove `lib/main.js`

This patch moves `lib/main.js` to `bin/crisp.js`.

Before this patch, `bin/crisp.js` was essentially just executing `lib/main.js`. So, `bin/crisp.js`'s source code might as well be that of `lib/main.js` (without the IIFE).

There's also the added benefit of `package.json`'s `"main"` field now pointing to `lib/crisp.js`. Now anyone can do

``` js
var crisp = require('crisp');
```

and start their own server programmatically. Before this patch, `require()`ing `crisp` would load `lib/main.js`, which would immediately invoke a bunch of command-line parsing (which is not what one would expect when they `require()` any module).