Remove unused acceptance tests and dependencies

Removes behat as a dev dependency, and the acceptance test directory from the source tree.

I've also added the remaining dev dependencies to the .gitignore file, so make the git output less noisy in dev setups.

Important note: While this change removes behat as a dev dependency, it introduces "ext-yaml" as one. The reason is that behat pulled in Symphony\Yaml, which was used by the integration tests to read database fixtures. Instead of reintroducing Symphony\Yaml, I went for the PECL php-yaml extension instead.