Add Gradle build support for Lambda HOL

[breskeby]

This adds a Gradle support to the lab, to be able to verify your results from the commandline.

with this pull request merged, you can:

• run all tests via ./gradlew check
• run all tests in exercises by running the testExercises task via "./gradlew testExercises"
• run all tests in solutions by running the testSolutions task via "./gradlew testSolutions"
• run a particular test in exercises by running "./gradlew clean testExercises --tests ex03*"

This uses gradle 2.14.1 which is the latest release gradle version. by using the gradle wrapper you ensure you always use the gradle version you tested this project with.