new create(ServletContext) method

If you call `DbUnitOperatorImpl.create()` (test env), the code uses `ServletContextHolder.getServletContext()`. In Grails 2.x you get a mock implementacion of ServletContext. 

With this new method you can inject the real ServletContext in `BootStrap.groovy` this way:

	DbUnitOperator.create(servletContext);

So `DbUnitOperatorImpl` will work with real servletContext in integration test and you will get real path for dbunit xml files.
Merge pull request #2 from amatach/patch-1

new create(ServletContext) method