Remove merge tags

[superbobry]

Merge tags, seem to be missing from the latest YAML spec, are you sure you want them in the library?

[goertzenator]

Merge tags along with other types are described here: http://yaml.org/type/

I use merge tags myself, and pyyaml supports a number of these extensions by default so I am inclined to follow in pyyaml's footsteps.

The one thing I don't like about merge tags is that I couldn't confine the implementation to the schema module; code leaked into yaml_compose.erl. Someday I want to take a closer look at pyyaml and see how it keeps things separated, and apply that to yamler if possible.