Minimatch patterns don't work when starting with "!"

Based on #382

```
var path = require('path');
var mm = require('minimatch');
var pattern = '!**/*.js';
var normalizedPattern = path.resolve(pattern);
var filePath = path.resolve('/some/path/to/file.coffee');
mm(filePath,pattern);
true
mm(filePath,normalizedPattern);
false
```