tasks.py: Typo in ec2_stop_instance documentation

docs say "start" instead of "stop"
resolves #12
resolves #12, add documentations
resolves #12 : adding security_group_ids for vpc
security_group_ids is optional
Update api.py

* Catch error caused by forgetting the argument on the ec2instance decorator, and give the user a hint.
Merge commit '0b6a0eaccc98adc3ad227a237e6a072f395ccced'
Merge commit '8a1ce01650c307c7b139cffa89a29ac27fc64947'