create set of fixed slots based off of numprocesses

allow reuse of slots as Process objects are reaped
Revert "create set of fixed slots based off of numprocesses"

This reverts commit aea929e7e6ca679b09d8fe4e0e2af08a747bc6d5.
supply a worker id that tracks w/ numprocesses

worker id's are reused as their corresponding process is reaped
a new property is added: "Watcher.nextwid" which returns the lowest unused worker id number
name change for nextwid

implied private property
removed unused var
Document the flapping plugin
Improve documentation for the stop command

 * fix title that was saying that this command stops the arbiter
 * reference the graceful_timeout option
 * various grammar fixes and wording improvements
Merge pull request #526 from amarandon/doc-fixes

Improve documentation for the stop command
Merge pull request #524 from amarandon/flapping-doc

Document the flapping plugin
Merge pull request #525 from mozilla-services/stats-tests

Stats tests
important typo (send SIGKILL and not SIGTERM)
Merge pull request #528 from thefab/final_stop_typo

important typo (send SIGKILL and not SIGTERM)
reorder import order in circus.util
remove import_module since we don't use it anymore
add coverage for circus.util
Move tests .ini files in sub folder

As per issue #468
Fix lines too long
Merge pull request #531 from amarandon/move-ini-files

Move tests .ini files in sub folder
Merge pull request #530 from mozilla-services/test-utils

Test utils
ensure wid re-use

- check max worker id
- calulate checksum of worker ids and compare against expected range of
  worker ids