getMinElement now supports values < 0.  Checked and shouldn't affect any uses.  subtract() from set with EOF, wiped out all negative values. added tests.
* Updated documentation for IntSet

* Reimplemented IntervalSet.subtract and IntervalSet.complement to operate over the complete range of supported values
* Expanded several methods in IntervalSet to operate on any IntSet
* Mark COMPLETE_CHAR_SET and EMPTY_SET as read-only
Merge pull request #623 from parrt/fix-set-subtract

getMinElement now supports values < 0.  Checked and shouldn't affect any...