Fix bug where items sometimes aren't deleted on Mac OS

Issue discussed here:
https://github.com/soffes/sskeychain/issues/11

Use SecItemCopyMatching and SecKeychainItemDelete instead of SecItemDelete to work around it.
Merge pull request #15 from noa--/pullreq2

Fix bug where items sometimes aren't deleted on Mac OS
Add tests and move files into source folder
Move changelog to https://github.com/soffes/sskeychain/wiki/Changelog
Bump license year
Add Gemfile *sigh*
Run bundle install on CI before running
Support Ruby 1.8
Add Test schemes
Merge branch 'master' of git://github.com/soffes/sskeychain into pullreq1