Added stripCloneAtts() method.
Renamed method to stripCloneAttrs()
Fixed false-negative attribute check bug.

The check now allows for "false" boolean attributes.
Return the clone
Built CS file
A more robust way to check if the model attribute exists

previously the clone attribute wouldn't get removed if set to `null` or
`undefined`.
Test case for #219.
Merge branch 'master' of github.com:spine/spine