WIP. Redo almost everything.

Read info about an existing repository.
WIP. GREEN Read package info starting from a repository.
WIP. Refactor.
WIP. GREEN. Throw an error if repository doesn't exist.
WIP. Remove unneeded private writers for entity attrs.
WIP. Add #repo? method.
WIP. Small refactoring.
WIP. Green. Throw error if package is not found.
WIP. Green. Add `Repository#package?` query method.
Add Contributor Code of Conduct.
WIP. Refactor Client so that it properly uses Bintray::API.
WIP. Red. Add `#add_package`, still being performed w/o authentication.
WIP. Add package creation.
WIP. Refactor Bintray::API.
Add licenses.
WIP. Replace hardcoded licenses with constants.
WIP. Make repository end-to-end test less brittle.
WIP. Red. Add end-to-end test for `#del_package` method.
WIP. Green. Add `#del_package` method.
WIP. Refactoring.