Remove modeshape-core's dependency on Apache Tika

modeshape-core has a compile time dependency on Apache Tika because it uses it for mime-type detection. Tika is a heavyweight dependency with a lot of transitive dependencies of its own. This has always been an Achilles heel especially when it comes to productizing Modeshape.

As such, we should keep the current behavior of using Tika for mimetype detection, but make it 100% optional from a runtime perspective, meaning that if Tika is not present in the classpath at runtime, ModeShape should fall back to an extension-based detection mode.