Define all util methods as module_functions

Potentially we can now mixin, but also this is a little cleaner
Add check_length method to Util

Also, add a specific class for that error
Also catch nil strings

And differentiate this from length, since nil is more indicative of
caller error (probably)
Fix nil error message
Util functions use check_length
Box and SecretBox use Util.check_length

Hopefully this will make the code climate slightly better.  Also, we
have standardised length errors
Make Keys use Util.check_length

Standardized errors ftw
Merge pull request #31 from cryptosphere/check_length

Centralize length checking into one method