Optimized sorting by letting HitProperty implement Comparable directly.
Added option to make concordances from forward index instead of
content store.

Off by default, because while more efficient, the resulting
concordances don't have any punctuation or XML tags.
We're working on adding the punctuation, though.
Added code to test the impact of the forward index I/O on
sorting/grouping.

The test code returns random context instead of actually reading context
from the forward index, so we can measure the difference in speed. It
turns out that sorting/grouping large result sets on context is clearly
I/O bound on systems without a lot of memory.