Remove Roo documentation from distribution zip

To reduce the size of the distribution zip due to the growing size of the documentation files, especially diagrams, the "docs" directory will not be added to the Roo distribution zip. The documentation is on-line anyway at http://static.springsource.org/spring-roo/reference/html/index.html (HTML) and http://static.springsource.org/spring-roo/reference/pdf/spring-roo-docs.pdf (PDF).