Backtrace In Errors

This updates the Adama backtrace to contain the backtrace of the original error with its own backtrace added to the end.

Interactor Style Command Rollback

This version updates command logic in two ways

• When a command fails it does not call its rollback method
• When an invoked command fails, the invoker does not roll
  it back (but it does roll back the commands prior to it
  that succeeded)

This follows from the idea that a command should have one responsibility so if it fails it does not need to be rolled back. This is how the interactor gem handles it: https://github.com/collectiveidea/interactor#rollback

We need this behavior for Adama to work as a drop-in replacement for Interactor

v0.1.1 validation adjustments

Merge pull request #3 from bugcrowd/version/0-1-1

Version bump 0.1.1