Drupy

This release introduces a new argument `--drush` that allows passing a custom drush executable.

* drupy now has an optional dependency on ruamel.yaml to support YAML files.
* [FIX] drupy can now handle tarballs/zip-files without a top-level directory.

* Changes drupal.org packages so that it only prepends the drupal.org package download if needed: Only if the build-pipeline starts with no other non-patch item.)
* Narrows down the pattern for drupal.org packages to only valid package / core-version / version combinations.
* Restricts `git --depth=1` downloads to build items without `revision`.

* FIX: Remove double-quotes from site-name.
* NEW: Support to shallow copies for git repos (`git clone --depth=1`). This is also the new default.
* NEW: Detect patched drupal.org projects: Now you can use `project-7.x-1.0+patch` and drupy will still download the package from drupal.org automatically. Also projects with the right format (`[project-name]-[core-version]-[version]+[patch1]…`) are now detected as drupal.org packages by default.