Add additional GroovyClassLoader tests

These tests specifically test for compliance with parts of JLS Section 5.
More tweaks to the documentation CSS for a more elegant typography title size scale
Problem of level offsets with asciidoctor
Adding Grant McConnaughey in the contributors list
tweak GroovyConsole to display more accurate generics information for extends/implements
Documentation for optional typing
Fix failing unit tests
Fix spread-dot operator in user guide.
Merge pull request #451 from pledbrook/doc-patch-01

Fix spread-dot operator in user guide.
Correct javadoc references to Collect.collectEntries().
Merge pull request #452 from pledbrook/patch-01

Correct javadoc references to Collection.collectEntries().
Introduction to type checking section
Add operator overloading section to Groovy specification.
GROOVY-5957: groovydoc does not create links for non-fully qualified classes in same package
tweak typing doco wording
Merge pull request #455 from paulk-asert/groovy5957

GROOVY-5957: groovydoc does not create links for non-fully qualified cla...
Merge pull request #454 from pledbrook/doc-patch-02

Add operator overloading section to Groovy specification.
Documentation of simple rules for type checking assignments
GROOVY-3680: Cannot use Groovy class with @Bindable or @Vetoable in Java code: cannot find symbol for addPropertyChangeListener
Added documentation for list/map constructors