Refactor to cleanup code base and fix various bugs.

Refactored the stapler trait, attachment, and storage drivers to fix a
bug when using STAPLER_NULL.  Events now fire listeners on the attachment (instead of inside the
stapler trait).  Deletes and writes are now pushed into queues that are
flushed after a save or delete.  Added listeners for the 'deleting'
event to queue files to be deleted.
Simplified file storage driver, various other minor tweaks and
improvements.
Fix README typos.
Swap out old resizer bundle with Imagine Image.

Swapped out the old resizer bundle that was being used for image
processing with the imagine image library.  Broke the attachment
validation functions into a separate class.
Add ability to provide callbacks in styles array.

Added the ability to provide callbacks as a style.  If a callable is
passed as the value of a style option stapler will invoke the callable
while also injecting in the current uploaded file object and the current
instance of Imagine.
Update readme file, minor bug fixes.

Updated the readme file to include more detailed documentation as well
as fix some typos.
Updated the s3 config file so that the default region configuration is
now set to US Standard*.
Update readme file.
Update readme file.
Update readme file.
Update readme file.
Add attachment destroy method, update readme.

Added an attachment destroy method for clearing attachments without
having to call the model's save method.
Updated the reame documentation.
Update composer.json keywords
Merge pull request #3 from CodeSleeve/development

Development
Fix merge conflict with rtablada-master
Fix typo in Readme file.

Fixed a typo in the quick start section of the readme file that was
using the wrong name of the stapler trait.
Update documentation.
Update readme.
Rename Validator to AttachmentValidator.

Validator has now been replaced with ServiceValidator in the stapler
service provider IOC bindings.  This was done in order to prevent
conflicts with the Laravel Validator library.
Fix bug configuration cascade bug.

Fixed a bug that was preventing published configuration options in
app/config/packages/codesleeve/stapler from being loaded.
Fix setting an attachment attribute on model.
Update the documentation.