Allow checking for exception equality and string comparisons.

I noticed this due to t/version_tag.t failing and blowing up.
is($@,"","chown wasn't supported in 2.13");
overload in 5.10 resets fallback each time its called.

Best to only call overload once.
Merge pull request #57 from schwern/bug/exceptions-overload-fallback

Allow checking for exception equality and string comparisons.