Add additional GroovyClassLoader tests

These tests specifically test for compliance with parts of JLS Section 5.
GROOVY-6840: Create new ArrayList instead of returning empty lists for JSON Parsing
GROOVY-6840: Fix Typo
GROOVY-6840: Add Test Case for Empty Lists
Added documentation for Statements
Merge pull request #448 from grantmcconnaughey/documentation

GROOVY-6268: Documentation on Statements
GROOVY-6868: Upgrade SLF4J to 1.7.6 and Logback to 1.1.2
Merge pull request #434 from kaendfinger/GROOVY-6840

GROOVY-6840: JSON: Use ArrayList for Empty Lists
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.