Merge branch 'contact-list-activity' into 'master'

Use a RecyclerView for the Contact List

This is the first RecyclerView in the project. I am using a SortedList to keep the contacts sorted by latest activity.

Also, I removed the icon footer and moved the icon into ActionBar. I did the same with the long-press contact deletion action which eventually will move to the new contact details activity.

Several icons have been replaced by vector drawables and all the views are now defined in XML.

See merge request !38