Added support for including excluded dependencies when generating POM file.

Added null check to PluginBuildSettings when no plugin dependencies are
present.
Added single dependency resolution logic to DependencyManager to support exclusion functionality when creating a POM file.
Merge pull request #453 from jdpgrailsdev/2.3.x

Added support for including excluded dependencies when generating POM file.
Merge branch '2.3.x'

Conflicts:
	grails-aether/src/test/groovy/org/codehaus/groovy/grails/resolve/maven/AetherDependencyManagerSpec.groovy