Corrected typo in exception name: 'InvaidPermissionError'.
Made the change backwards-compatible & added a spec.

We don't want to hose anyone who's already catching 'InvaidPermissionError', after all.
Made the spec more elegant.
Merge pull request #59 from fimmtiu/fix-typo-in-exception-name

Corrected Typo in Exception Name InvaidPermissionError