added two new parameters before deployment: build_before and build_after...

This adds two new parameters that can be used in your project's ``conveyor.yml``:
- build_before
- build_after

These parameters will give you the build hash that was on your target before and after deployment respectively.

This can be useful if you want to hook into certain scripts on your target that can notify webservices of the deployment made and use these hashes to give more detailed information.