GROOVY-5839: add test case to avoid regression
Add a cobertura task
GROOVY-5873: generalize generics application and extend to be used on fields
Tried to make the test more robust on the buildserver and to speed it up for faster machines.
Document limitation of normal map-style constructor use when using the Immutable annotation.

See http://groovy.329449.n5.nabble.com/Unexpected-behaviour-when-using-Immutable-on-a-class-with-java-util-Map-field-tp5716170.html
Merge pull request #219 from jwagenleitner/immutable-javadoc

Document limitation of normal map-style constructor use when using the Immutable annotation
Added missing "." at the end of new javadoc for GROOVY-6269.