Refs #25 -t  Don't recommend the -l option
- bugfix: should not print `null` if no contents
- feature: support multiple contents for one element tag
- feature: revised attribute's null to print wihtout assignment that help for support angular.js' directive
- revised attribute undefined to have same behavior like attribute null
Clean up pull request from @paynechu:
  Support attributes with no value if value is null or undefined
  Don't print 'null' for empty raw tags like <script>
Merge branch 'paynechu-master'
teacup@0.2.10
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/teacup.js
	src/teacup.coffee
	test/tag.coffee