Add ResolvePrivateTag method for private tag lookups.

Private data elements are mobile, which means that different data
sets might store them at different locations.  Given a private
data element which has a tag of (gggg,xxee) according to its
dictionary, the first two element digits "xx" can change from one
data set to the next, and the data set must have a "Creator Element"
at (gggg,00xx) to allow the digits of "xx" to be resolved.  Please
read DICOM Part 5 Section 7.8 for additional information.