Set target and source compile options

[tastle]

It would be nice to set the target and source attributes on the javac tag to ensure that this library is always built for a version of Java that the community at large can use.

I discovered during the last build that these weren't set and accidentally built binaries that were only suitable for Java 7.

I've attached the updated build.xml file with 1.6 set for source and target for the source and unit tests. You may be able to get away with 1.5 or 1.4 depending on language features used.