Add jdk6Test task

# Note about the commit

If someone does not have jdk6Home configured the tests will be skipped. This may not be ideal, but I felt it was better than requiring local developers configure the property. If this is not the case, let me know and I can make jdk6Home a required property so that we can always run the jdk6Test task.

/cc @cbeams @jhoeller @philwebb

# Commit Message

Previously the build was unable to build against JDK 7, test against
JDK 7, and test against JDK 6.

This commit allows setting of a gradle property [1] named jdk6Home.
Upon setting the jdk6Home property tests will be ran against JDK 6 as
well. One option for setting the property is to include jdk6Home
property in ~/.gradle/gradle.properties. For example the content of
the file might look like:

    jdk6Home=jdk6Home=/usr/lib/jvm/jdk1.6.0_32

An alternative is configure jdk6Home from the commandline as shown below:

    gradle build -Pjdk6Home=/usr/lib/jvm/jdk1.6.0_32

[1] http://gradle.org/docs/current/userguide/tutorial_this_and_that.html#sec:gradle_properties_and_system_properties