Add support for observing other objects
Merge master into develop
Add convenience methods for observing other objects

Observe multiple key-paths at once or observe using selector.
Redirect existing self-observing methods to use these.
Declare new block types for observing foreign objects.
New block type names, some comments fixed
Add macro for observing single property, since this is the most used case
Merge branch 'master' into 'develop'
Consolidated comments from previous merge
Foreign observation block refactored, now have weak self parameter
Fix possible mutable key-path by copying it
Create first tests
Updated README and LICENSE
Merge branch 'release/v2.1'