[fix] in app.sandbox.stop, when sandbox can't be stopped directly through _ref, validate if the given string is a valid selector first to prevent Sizzle throwing out error.

Also added check before cleaning up to prevent tests from failing
[fix] use $.find instead of $() to prevent unnecessarily create jquery instance
Merge pull request #9 from Lulo/stop-sandbox-ref

[fix] Validate selector in app.sandbox.stop