improve comments, code in model, language tests
remove obsolete comment
fix mis-indented code
reformat/minor code improvement
split ChecksumReport off from TestChecksums
code improvement in ChecksumReport
avoid SynchronizedMap (dodgy, will be deprecated in 2.11)
redo ChecksumReport using a Scala parallel collection

instead of raw ExecutorService stuff
code improvement in TestChecksums
have TestChecksums complain if too many version mismatches

fixes #515
be even more strict about model checksum version mismatches

don't just count ignored failures, count *all* version mismatches.  we
just really shouldn't let more than a handful of models have outdated
revs, if we want to catch engine/compiler regressions. the models
don't change that much these days so it isn't a big burden

further addresses #515
convert another piece of our build to .sbt format

because it's awesome. haters gonna hate
make `ts` (TestChecksums) take args

so you can do e.g. `ts GenDrift` to run just the GenDrift models
add comment and `extends` to SlowTest

the `extends` enforces the intended usage. not sure why we
didn't have it before
add comment
Bring HorizCyl and VertCyl in line with Box/Torus!
Merge pull request #520 from NetLogo/headless-tortoise-box

Bring HorizCyl and VertCyl in line with Box/Torus!
simply omit unwanted models from test/checksums.txt

now that checksums.txt is in this repo and not the models repo,
there's no need to exclude entries programatically

(this also fixes an issue where the unwanted models were excluded
from TestChecksums but not from ChecksumReport)
upgrade to sbt 0.13.1 final
rearrange some code a little for clarity