Fix globbing in Gruntfile for 'grunt dev'

Due to a missing globbing-pattern, the dev-task would not watch any directories
below 'css'. Instead, it would only react to changes immediately within 'css'.
The '**' pattern includes the current as well as all subdirectories and should
fix #5.
Merge pull request #6 from baumanno/master

Fix globbing in Gruntfile for 'grunt dev'