remove Ant support
remove doWithWebDescriptor and load Grails programmatically via Servlet 3.0 APIs. Also allow plugins to hook into startup via Boot's ServletContextInitializer
This commit includes several significant changes that align Grails closer to Spring MVC and Spring Boot:

1) UrlMappingsFilter is now gone and instead represented by a UrlMappingsHandlerMapping and UrlMappingsInfoHandlerAdapter combination
2) With the filter gone all requests now go directly onto the GrailsDispatcherServlet
3) The custom GrailsDispatcherServlet and hideous SimpleGrailsControllerHelper classes are gone, their functionality being handled by the Spring MVC DispatcherServlet and the above mentioned handler mappings

The benefits of these changes is that Grails applications appear even more like a regular Spring application now with very few custom extensions added to the DispatcherServlet.

One remaining TODO item is to rewrite ErrorHandlingServlet for these changes.
delete tests that were testing removed classes / fix compilation errors
add web initializer jar to default BuildConfig
introduce now boot subproject