Set or override system properties from a specified file

See #16 

This is invoked before TestPackage itself is completely initialised so that, in theory, you could use a file to set system properties for TestPackage itself, rather than passing them all as command line arguments. Using a capitalised 'P' for the alias for consistency with other common Java projects that do this.

Note the use of some Java 7 syntax, such as the try-with-resources statement; not sure if you're happy with this given your Travis config includes JDK6 (hint!).