simplified ruleset configuration

- only allow a single ruleset file instead of multiple
disabling file field if "Use custom ruleset" button is disabled

- it was correctly disabled in PreferencePage.initialize() but then
  enabled again in FieldEditorOverlayPage.updateFieldEditors()
validating custom ruleset file during configuration

- trying to parse the selected file in project/workspace properties
  right after selection to detect if it is a valid ruleset file
explicit invalid ruleset file handling in CheckCode job

- this provides a cleaner error message in the dialog shown to the user
upgraded to CodeNarc v0.21.0
Merge branch 'master' into development
make project build with Java 8

* upgrade of groovy-eclipse-batch required to support building with Java 8
* otherwise this error occurs:
  [ERROR] The type java.util.Map$Entry cannot be resolved. It is indirectly referenced from required .class files
* ATTENTION: dropping support for Groovy 1.7
  * for details see commits
    * 5ba9bb64ed8b70d2a18e96d3efdc956c3b9ba071
    * cf87dd9fd43c5044852e480894373fc674a37e9a