post save hook for decrypts documents

When we were saving documents, we immediately had to decrypt afterward since it would be encrypted.  I added a post save hook to take care of this.  It was a little messy since post save hooks are only fired on the parent documents, not on sub docs.  

Changed tabs to spaces in test.  View the PR changes at https://github.com/cinch-financial/mongoose-encryption/pull/3/files?w=1 so you don't have to deal with the whitespace changes.
should check for `decryptSync` since that's what you're calling down below