Fix #21 by moving win32 CFLAGS tweak above other test compiles.

Credit to @phasis68 for the patch.
Allow forcing fallback impl.
Add some missing license headers.
Make test work under 1.8-compat.
Add numeric CAS logic for fallback, MRI, and Rubinius.

Note that this impl is pure-Ruby, so MRI and Rubinius will have
slower CAS than JRuby. A pure native impl is welcomed.
Version 1.1.8.
Limit gem description to just commit log.
Add license to gemspec.