Add #:include directive. Implement circular reference guarding and sanity checks.
Merge branch 'master' of https://github.com/kivy/kivy into kv_include

merge master before pull request
Correct example
Add option to force-include which first unloads the file and then reloads it. Check for proper extension and inclusion status before any disk access takes place. Update example.
Update version added to 1.8.1 and remove accidental ambiguity with the inclusion list as per akshayaurora
Merge branch 'master' of https://github.com/kivy/kivy into kv_include
Merge pull request #1720 from techwiz24/kv_include

Add include directive to KV Language specification