Added compilerComplianceLevel method, and classpathVariables method.

Author: scottresnik

src/main/java/com/diffplug/gradle/oomph/ConventionJdt.java

@@ -45,9 +45,8 @@
  *             markDefault = true // or false
  *             executionEnvironments = ['JavaSE-1.6'] // any execution environments can be specified here.
  *         }
- *         compilerComplianceLevel(JavaVersion.VERSION_1_6)
- *         //or
  *         compilerComplianceLevel('1.6')
+ *         classpathVariable('myClasspath', '/var/lib/repo')
  *     }
  * }
  * ```