Move authentication support into core

[cjstehno]

The BASIC and DIGEST authentication support should be converted to config properties rather than "features"

• the feature mechanism should be deprecated and removed by 1.0 with a migration strategy for existing code (probably just HttpBuilder-NG)
• it would be nice to be able to define BASIC, DIGEST and non-authenticated expectations on the same server but this would not be a requirement

Add something like:

def server = new ErsatzServer({
    basicAuthentication true
    digestAuthentication true
})