Removes encryption and decryption, adds some error handling

Removes encryption and decryption from SessionCookie.  The encryption
and decrption will be handled elsewhere and can be enabled or disabled
via the cookies.encrypt config setting.

This fixes flash messaging/SessionCookie decoding failure when
cookies.encrypt = true
Fixes \Slim\Slim::setCookie() method signature error.
Whitespace fixes
Fixes travisCI PHP 5.3 test failure (build 299).
Merge pull request #635 from jeremykendall/fix/cookie-encryption

Fixes flash messaging/SessionCookie decoding failure when cookies.encrypt = true