Encoded urls

This is a small change to the latency app that will allow us to generate a list of random response urls that will consistently behave the same.

I opted to just use simple base64 encoding, as we don't really care if the data is seen. This is easily changes to an encryption/decryption module.

Note: It would be interesting to store the results of the sample call so we can verify that the correct responses were returned.

Major Changes:

- Converted url handlers from functions to struct types to make url encoding and decoding configurable
- Changed encoding of urls to randomly assign status code, messages, and delay
- Changed decoding of urls to read base64/json values


Example Url:
```
http://localhost:8080/json/eyJtZXNzYWdlIjoic3VjY2VzcyIsImRlbGF5IjozLCJzdGF0dXNfY29kZSI6MjAwfQ==
```

I suppose it's possible - but `strings.Split` will always return a slice of at least size 1, which would make the index `0` in that case.

It wouldn't do what we wanted in that case, but it wouldn't panic either.