update GitHub plugin version (API change - broken with 0.7)
drop execution environment to Java 6 where not needed

This keeps Java 7 requirement on isabelle.pide and isabelle.pure only at the moment, which depend on Java 7 libraries. Because of dependencies, if running on Java 6, the main Isabelle plugins are not loaded, however the product manages to load correctly and avoids causing ugly appearance for Isabelle/Eclipse standalone.
new plugin for diagnostics with Java 1.5 requirement

Low Java runtime requirement to ensure that it is always loaded - to always report bad java version.
check for Java 7 and open a dialog if not available (issue #69)

The plug-in is auto-started and checks the Java version on startup. If invalid Java version is used, a dialog is displayed explaining that Isabelle/Eclipse will be disabled, plus some information on setting Java runtime.
Merge branch 'java7-report' into develop