Add FileNotifier

FileNotifier writes guard notification results to a file. This allows
users to handle notifications in a more custom way (think guard-shell).
Change work dir to the `watchdir`.
Document FileNotifier
Ruby 1.8 hash syntax
Add changelog entry
Merge remote-tracking branch 'origin/pr/407'

Conflicts:
	CHANGELOG.md
Double quotes are needed for escape sequences. [ci skip]