Groovy Site

Now, this is the groovy official site.
Exclude exclusions in AetherExcludeResolver and add tests for handling of exclusions of unavailable artifacts

See GPRELEASE-59
triggering ConvertersConfigurationInitializer moved to spring init method
refactoring: using GrailsApplicationAware interface for grailsApplication bean injection
changing integration spec template to create class that extends IntegrationSpec class
changing integration test filename pattern to *IntegrationSpec (it wont collide with creating unit test for the same class)
use https to access Maven Central
GRAILS-11625 - improve handling of @Validateable(nullable=true)

If a class is marked with @Validateable(nullable=true), constrained properties which do not express a nullable constraint should be configured with nullable:true.
Merge branch '2.4.x'
temporarily remove failing test
Revert "temporarily remove failing test"

This reverts commit 40ba088c1f1b5d18ef8bce170444518c45cfe5b2.
introduce AsyncController trait
remove ControllersAsyncApi
introduce converters extension api
reorganize how ConvertersConfigurationInitializer is initialized
The ConvertersConfigurationInitializer should already be initialized by Spring
fix failing tests

Some of these tests don't appear to really need to interact with the ConvertersConfigurationInitializer. Pending investigation.
simplify tests
remove metaprogramming from tests

These tests should be picking up org.grails.web.converters.ConvertersExtension methods
troubleshooting test failure on CI server