Allow forward-slash in Twig namespace (fixes #6)

Keeps a cache of added namespace keys mapped to valid twig namespace equivalents, a little messy, especially catching the exception and replacing.

I can see where you were going with the colon namespace separator, however, you would still need to replace the forward-slashes before the colon and replace namespace the exception message for consistent error reports.

P.S. merge relies on #7, can change if required.