Correctly memoize our knowledge of the existence of stty

Three states are required for memoizing whether stty exists: yes, no & unknown. The prior implementation does not distinguish between the no & unknown states. On Wiindows, where stty is not likely to exist, this results in #stty_exists? rechecking every time for existence of stty.
Replace use of '/dev/null' with DEV_NULL throughout the gem

DEV_NULL is a constant that is evaluated at load time. On Windows, this constant is set to 'NUL' on linux, it is set to '/dev/null'
Fix grammar
Code around apparent bug in Thread#alive?

The documentation for Thread#alive? states that it "Returns true if [the thread] is running or sleeping." In ruby 1.9.3p429 (2013-05-15) [i386-mingw32], it also returns true when the status is 'aborting'. Explicitly checking for states fixes this bug.
Merge pull request #461 from cablegram/fix-grammar

Fix grammar
Merge pull request #462 from cablegram/thread-alive

Code around apparent bug in Thread#alive?
Merge pull request #463 from cablegram/windows-fix-dev-null-2

Replace use of '/dev/null' with DEV_NULL throughout the gem
Merge pull request #460 from cablegram/windows-fix-stty

Correctly memoize our knowledge of the existence of stty
Update CHANGELOG [ci skip]
Fix rspec 2.14 warnings
Fix spec