improve handling missing credentials by fixing missed configuration validator checker

removed the configuration checking code from the parse function of the
provider class AwsS3Provider to the helper class CdnHelper
and update the validator to check for any required configuration if missed
and throw an exception

Issue: #10
add configuration validator class

- remove configuration validate from helper class and create a configuration validator class
to validate the configurations
- rename $required_configurations to $rules in the amazon provider class

Fix #12
update read me file

- fix the small styling problem
- add link to packagist
fix class description
Merge pull request #12 from Vinelab/fix/upload-with-missing-credentials

improve handling missing credentials by fixing missed configuration vali...