now script is using compiler from "$project.retrolambda.jdk/bin/java"

If system's default JDK isn't 1.8 than retrolambda had problems.

Java task was completely unaware of *retrolambda.jdk* option and adndroid on only used library from JDK8, but if compiler was from JDK7 it failed output 1.8 class files.

Now both compile task and retrolambda tasks are running using JDK8 in separate process, if we have different JDK running gradle.
include section cleanup
refactored checking if run from java8
cleaned up imports/print statements
Finds java paths to properly run compile and tests with correct version

java 8 can be set with either retrolambda.jdk or JAVA8_HOME
java 6 or 7 can be set with either retrolambda.oldJdk or JAVA6_HOME/JAVA7_HOME

error messages are also improved