Remove shell-executer
Add open4 as a runtime dependency
Add new Executer class

This replaces shell-executer
Use Executer in Instance

Use the new Derelict::Executer class to execute Vagrant in
Derelict::Instance.
Update references to shell-executer in README
Update references to shell-executer in comments
Merge pull request #13 from bradfeehan/new-executer

Avoid leaving Vagrant processes running when receiving an interrupt signal