change Twilio::Config internals to module attrs

Fixes #26
http request methods do explicit check for config setup

This was not necessary previously because the code relied on a
const_missing being triggered when the config was not setup. This
worked because the old config credentials were stored in constants.
find/replace of account_sid and auth_token consts w/ config methods
replace old spec config teardown with new syntax
add spec for ConfigurationError when auth_token is not set
don't over-ride config set up in outer context
Config getters raise ConfigurationError exceptions when not set

hat-tip to @stevegraham for the idea (and code) :)
Merge pull request #27 from ajsharp/issue-26

Change configuration internals from constants to module attrs