Formats error message when YAML file is invalid.

[Fixes #4]
Displays time elapsed to create/update a Stack.

[Finishes #5]
Merge pull request #10 from cotap/time-elapsed-update-create

Displays time elapsed to create/update a Stack.
Merge pull request #9 from cotap/error-yaml-file

Error yaml file
Merge pull request #11 from cotap/stacker

Stacker