Added a --versions option to `bower list`

This would be useful for programs that need to programmatically determine which versions of packages have been installed (instead of trying to parse the regular string output of `bower list`).

An example:

```
$ ~/dev/src/bower/bin/bower ls --offline --versions
{
  "jquery": {
    "path": "/private/tmp/bower-testing/components/jquery",
    "version": "1.9.1"
  },
  "jquery-zoomer": {
    "path": "/private/tmp/bower-testing/components/jquery-zoomer",
    "version": "95df92ade3e581b9782be5ac6556c8db9b5c1aab"
  },
  "messenger": {
    "path": "/private/tmp/bower-testing/components/messenger",
    "version": "1.3.2"
  }
}%
```