feat(superagent): set superagent as an external dependencies

This idea is to enable overriding the agent and also to fix the ugliness of
the hack due to es6 tranpilation on a default property: {default: something}.

BREAKING CHANGE:
remove superagent-e6 dependency
refactor(tests): upgrade to superagent being externalized
chore(karma): remove superagent-es6 dependency
Release 0.6.0