v1.4.0

NOTE: v1.4.0 contains a lot of breaking changes - please read the readme

This is a major release that has a lot of breaking changes.
All changes came from community requests, I took all the PRs and Issues discussions and compiled them into this new version.
If anyone has anything to add/comment this is the place.

• Move all cache logic to ImageCacheManager
• Introduce ImageCacheProvider - a top-level component that provides an entry point for customization over the ImageCacheManager
• Separate concerns of cache and storage.
  • MemoryCache manages the caching of URLs, this means that for each URL there is an entry in the cache with its expiration date, local file path, etc. - all decisions are made based on the url cache layer.
  • fs layer has no knowledge of the cache layer and is orchestrated by the ImageCacheManager
• Use ES6 classes instead of the deprecated React.createClass
• Use prop-types instead of the deprecated React.PropTypes