More aggressively shrink jar size

[jcrist]

Currently skein.jar clocks in at 13 MB, and that's with minimizeJar set to true for maven's shade plugin. However, many of the classes/files included in the packaged jar are unnecessary, and are only included due to reference cycles among them. With some work we should be able to determine at least a subset of these files that can be safely removed (e.g. guava.gson should be unnecessary). Additionally, there are some non-class files that should be fine to strip (e.g. the raw proto files in the protobuf compiler).