Use JDK 1.7 for building, but use 1.6 compatibility

We should switch to using JDK 1.7 (OpenJDK?) for builds, and should set the source and target versions to 1.6 so that ModeShape 3.0 can continue to be used in 1.6 environments. Building with 1.7 will also allow us to implement the methods added in JDBC 4.1 (to throw a proper error message), which mean that ModeShape 3.0 can be used in 1.7 environments without seeing AbstractMethodError indicating a missing method.