Cleanup emitter interface

* Allow to instantiate emitters without emitting into buffer
* Add idempotent write_to_buffer (only writes once)
* Fix bugs where parent wasnt set to Emitter instance

This changeset enables an upcoming nice refactoring for terminated
expressions.
Elimiate whole class of "send to unterminated expression" class of bugs.

* Add a whitelist mechanism for "terminated" expressions.
* Let the child emitter tell if expression needs to be terminated
  explicitly
* Closes #8 and possible all non reported siblings
* Handle regular sends via Emitter::Send::Regular
* Remove lots of ungly code that deals with inserting parentheses
* Drop 1.8 support!
Bump version to 0.1.0
Remove 1.8 only emitter
Add a note about 1.8 support
Adjust flog score down
Merge remote-tracking branch 'origin/master' into preserve-comments

Conflicts:
	lib/unparser.rb
	lib/unparser/emitter.rb
	lib/unparser/emitter/send.rb