implement addHeader and addHeaders
do not use emtpy() method return
revert Guzzle to its initial definition
set the flash variable in the view
add unit tests for the flash object
don't explode headers
remove output buffering
test response send
update guzzle
allow passing array of methods to Slim\Route::via() and Slim\Route::appendHttpMethods()
Switch to composer autoloader for Phpunit

Autoloading can be fully delegated to composer and the bootstrap file
for testing can be replaced by it.

This only needs to remove one test that already looks superfluous as it
tests the Slim autoloader  but composers autoloader is used and we can
expect that the composer autoloader does work. And if not, it's not an
issue in Slim.

Thanks to that test removal, the class Foo can be removed as well.

Additionally the php ini setting from bootstrap.php to disable session
cookies as otherwise headers would be sent has been moved into the
phpunit configuration.