java: Add new Gradle-based template assignment

To use this template to create a new assignment,
simply cp _template to the name of your new assignment
and drop in the appropriate tests.
java: Update current exercises to use Gradle

Gradle can be run directly on Mac/Linux/Windows and
auto-download dependencies (much like sbt for Scala),
as well as simplify IDE integration.

Add appropriate .gitignore for this sub-tree.

For developers, to add a new exercism:

    cp -r _template new-exercism
    # create test in new-exercism/src/test/java/SampleTest.java
    git add new-exercism

Users will be able to just run "./gradlew check" to run tests.
java: Update documentation with cmdline and IDE docs