polishing
deprecated SourceSetContainer/ConfigurationContainer.add methods in favor of create methods

- changed usages of add methods to create methods
- ported DefaultScriptHandlerTest to spock spec
ported JUnit/JMock test to Spock
Fixed wrapper to deal with a failed distribution download.
Split up the wrapper int tests and added some coverage for failed dist download.
Moved the check for the wrapper jar size to live in the wrapper int tests. Also fixed the check so that it looks at the correct jar file.
Prefixed field name with '_' to hide it from Groovy.
Include Groovy 2.1.0 in the set we test compilation against.
Tweak to layout.
Some minor spec updates.
Ignore this test on the IBM JVM as well for now.
Some spec updates.
UnknownModelException is no longer thrown by ProjectConnection.model(), as it's too early to tell. Document this in the release notes.
Javadoc tweak.
REVIEW-1888: verify correct jacoco version is used in integTest.
REVIEW-1882: add testcase for package success rate
Add missing import.
Fix test assertion to correctly check for only one processing having downloaded the wrapper.

Previous version would always pass.
Groovification of test, no functional change.
Add xavier to the external contributions list in release notes.