Allow steal to live in another folder

Steal has always enforced a folder structure that follows this formula:

> stealconfig.js
> steal/steal.js
> steal/dev/dev.js

Due to the popular of JavaScript tooling and new structuring techniques, many people now would like to move steal into a subfolder somewhere like lib/ Or, for example, people who use Bower will, by default, have steal in bower_components/steal/ (when Steal is in Bower). This pull request allows Steal to be arbitrarily placed in any folder and during the build step this will be set. If used from the command line it can be set like:

```shell
steal build --steal-dir lib/steal path/to/app.js
```

Or if using the API:

```javascript
var build = require('steal-tools').build;

build('path/to/app.js', {
	stealDir: 'lib/steal'
}, function(){});
```