Add JUnit jar for testing

This feels like cheating, but it's the only way (so far) that I've
been able to get Ant to find JUnit in Travis CI. I'm hoping that I
eventually come up with a better solution.

For reference, see: http://stackoverflow.com/a/18651699
Add and configure build.xml for testing
Add .travis.yml
Force LabelQueryJSOTest#testGetSingleId to be run only in production mode
Use a tolerance in ValueJSOFloatTest

Test that floating point values returned from ValueJSO#getDefault are
within a tolerance. I was incorrectly trying to match a float value
exactly. (I should know better.)
Merge pull request #18 from mdpiper/master

Configure project for testing with Travis CI