Add support for placeholders in hosts

I've added basic support for placeholders in hosts but arguments validation is not yet implemented. It should cover most use cases and if someone feels like adding validation, they can improve on this work. I tried to cover as much as I could provided I don't need this functionality myself yet.

And this PR really fixes #74 this time.

I'd like to rename `hosttokens` to `hostTokens` in some places, but we'll need to either provide our own serialization normalizer or switch to JMS Serializer because the normalizer behaves differently in Symfony 2.0. What do you think?