add common generator helpers.

This is kind of a flaky idea, but thought it could get some feedback.

Instead of every generator defining:

```
var guy = '\n     _-----_' +
'\n    |       |' +
'\n    |' + '--(o)--'.red + '|   .--------------------------.' +
'\n   `---------´  |    ' + 'Welcome to Yeoman,'.yellow.bold + '    |' +
'\n    ' + '( '.yellow + '_' + '´U`'.yellow + '_' + ' )'.yellow + '   |   ' + 'ladies and gentlemen!'.yellow.bold + '  |' +
'\n    /___A___\\   \'__________________________\'' +
'\n     |  ~  |'.yellow +
'\n   __' + '\'.___.\''.yellow + '__' +
'\n ´   ' + '`  |'.red + '° ' + '´ Y'.red + ' `\n';
```

I thought we could keep that within the generator itself, allowing them to access it with just `this.yeoman`. I created a special `lib/common.js` file for it, but couldn't immediately think of other similar common things that could go there.

Any ideas, or :bomb: this idea altogether?