Notification Rich Text Support

#### Notifications: Rich Text Support

Now Text and Comment notifications have support for inline images. We've implemented a simple component, *RichTextView*, based on TextKit.

The original goal was to wire *RichTextView*'s delegate to asynchronously download the images. However, that breaks if the image size is originally unknown: the user may / may not specify the embedded image size, beforehand.

For that reason, a simple helper class, *NotificationMediaDownloader* was implemented: this class receives a list of URL's, and will hit a completion callback whenever the assets are downloaded.

Duplicate downloads are prevented, and a simple cache has been implemented.

Closes #2661 

/cc @aerych + @dan may i bother you with a code review?. 
Thanks in advance!
I'm not sure about this. I think the view owner should be the one to decide whether to implement autoresizemasks or constraint based layout.  If its necessary that the view not translate autoresize masks, then it should implement `requiresConstraintBasedLayout`, but my preference would be to let the view owner make that call. 
https://developer.apple.com/Library/ios/documentation/UIKit/Reference/UIView_Class/index.html#//apple_ref/occ/clm/UIView/requiresConstraintBasedLayout