Removed substitutions for base64url decoding

Node handles base64url decoding natively: https://github.com/joyent/node/blob/master/src/string_bytes.cc#L154
The ":" to "/" didn't really make sense in a context of the spec or the encode function. Was this a typo that went uncaught due to the native decoding of any remaining underscores?
Thanks... mind adding another test that uses a larger string?  Perhaps read in a file and encode/decode it and compare the results?

Something like this:

```js
          it('works on larger input strings', function (done) {

                var input = Fs.readFileSync(Path.join(__dirname, 'index.js')).toString();
                var encoded = Hoek.base64urlEncode(input);

                expect(encoded).to.not.contain('+');
                expect(encoded).to.not.contain('/');

                var decoded = Hoek.base64urlDecode(encoded);

                expect(decoded).to.equal(input);
                done();
            });
```