Updating package.json

Updating package.json to set the main property to ./lib/assemble so we're loading the lib instead of the gruntfile.

This allows us to use assemble in other places by requiring it directly...

```javascript
var assemble = require('assemble');
var frontMatter = assemble.Utils.FrontMatter({});
var results = frontMatter.extract('./path/to/file.hbs');
console.log(results.context);
```

Better naming and conventions for the assemble Utils is another issue that we're working on.