Glide should not generically cache Uris

[sjudd]

Although it's unfortunate, content at uris can change without the uri changing so we can't reliably invalidate cache files. We can get around this for media store uris only by requiring users to pass in additional data from the media store (see http://bumptech.github.io/glide/javadocs/latest/com/bumptech/glide/RequestManager.html#loadFromMediaStore(android.net.Uri, java.lang.String, long, int)). This data must be available at the time the request is made so that we can construct the key for the memory cache synchronously.

To fix this, we should:

1. Default to DiskCacheStrategy.NONE for Uris (other than http/https).
2. Default to not caching loads for Uris in the memory cache.
3. Provide a nicer api for loading a Uri with some kind of cache buster string that can be mixed into the cache key for those who want to take on the responsibility of handling cache invalidation themselves.

See #172 for an example of where our current behavior can cause problems.