Memcached rails updates

CHANGELOG:
-Add travis configuration file (tests rubies 1.8.7, 1.9, 2.0, and rbx 1.9 mode).
-Fixes for unit tests breaking under Ruby 1.8.7.
-Updated gemspec to add check for signing key so that we can also build unsigned gem.
-Add travis build status image to README.rdoc.
-Fix Memcached::Rails write to allow nil options to be passed.
-Fix Memcached::Rails fetch's default options parameter to be nil.