Added support for System Property "yoga.jackson-library". Setting this property to "jackson" or "jackson2" will allow the user to force which Jackson library to use.

If the System property is not set, it will use whichever version it can find in the classpath. If both versions are found, it will use Jackson2.
Merge pull request #245 from cepage/classUtil

Added support for System Property "yoga.jackson-library"