Add image search functionality

[mattporritt]

Extend plugin to support indexing and searching of images.

AWS has a service that will return content information of a provided image. Integrate this service with the plugin so images can be indexed and searched.

Relevant documentation links: http://docs.aws.amazon.com/aws-sdk-php/v3/api/api-rekognition-2016-06-27.html#detectlabels https://aws.amazon.com/rekognition/

Initial high level tasks

• Add configuration options for AWS credentials and region for Rekognition service
• When files are indexed if a file is JPEG or PNG with dimensions greater than 80 x80 pixels send to recognition for processing.
• Use returned metadata to create index document
• Add document to index.

[mattporritt]

This is ready for testing now! have a look at branch: issue15-imagesearch for the test code.

[mattporritt]

This is merged to master now.