Update the build to allow setting source/target for the java compiler.
This is to allow 1.6 and 1.7 support in the same build.
merge tjoneslo: Update the build to allow setting source/target for the java compiler.

Merge pull request #17 from tjoneslo/pgjdbc/target-java-6
https://github.com/FoundationDB/sql-layer-jdbc/pull/17