Add Azure Face functionality

[Webreaper]

Enhance face-detection by calling Azure Congnitive Service Face API for images with a face detected in them. Enhancement for #14.

Add options for filtering when Azure is used, to manage the 30k per month free limit:

• Scan all images (for people with money or small collections)
• Scan only images where Accord face detection found a face
• Scan only new images (i.e., don't scan the previous existing 500,000 images)
• Scan images with certain tags (see Tag Attributes #154)

Also, provide the ability to batch-scan an entire folder, date range, tag etc.

Add AI source to image objects - separate face table? Add other attributes.

[Webreaper]

Use new PersonDirectory functionality to auto-train when using Azure Face Services.