Allow override of stylus or less variables.

When using stylus or less it should be possible to put override files into the app's styles folder. For example copying variables.less from derby-ui-boot/node_modules/bootstrap/less should allow the user to override paths. One useful thing to do would be to change the iconPaths by changing the variables.less to use:

```less
@iconSpritePath:          "/img/glyphicons-halflings.png";
@iconWhiteSpritePath:     "/img/glyphicons-halflings-white.png";
```

and then copy the pngs to public/img.