Move JacksonUtils and JsonLoader to their own package

Prepare for move into jackson-coreutils.
JsonPointer untangle, part 1: remove some JsonReferenceError references

The ultimate goal is to have JsonPointer completely independent, exception-wise,
from Processing{Exception,Error}.

Start by modifying some javadoc.
JsonPointer untangle, part 2: change ReferenceToken null handling

Use Guava's Preconditions.checkNotNull() instead of JsonReferenceError. Change
tests accordingly.