Add option to specify the path to the static directory

It is a pain to have to start the collector/evaluator from the cube bin directory.
This is especially true when you have a custom config.  This small change allows you
to define a "static" option as part of your config to define the path to the cube
static directory.

Usage:

```javascript
var config = {"static": "../node_modules/cube/static"};
var server = cube.server(config);
...
```