Track single-valued fields in a HashSet.
Add "java" to the list of prefixes excluded.
Create fall-back to creating multi-valued fields.

Just a note: Multi-valued fields are already created from the single-
valued fields via copyFields in the schema
Revert "Create fall-back to creating multi-valued fields."

This reverts commit 9399b4006bc120c2d858856ac740d7bbdc766c16.

On second thought, this likely isn't necessary, so let's skip it
for now.
Merge pull request #38 from adam-vessey/UMKC-64

Change how we decide to create single-valued fields in MODS "slurp".