Initial commit.

Just the output of `bundle gem bintray`.
Cleanup gitignore and Gemfile.
Rename license file.
Setup gemspec.
Setup Rakefile.
Documentation.
Ignore direnv rc script.
Setup test helper.
Walking skeleton.

System can create a new package into an existing repository.
Return a proper `Package` object after creation.

When a package is successfully created on Bintray, a `Bintray::Package`
is returned.
Configure Travis build.

Also specify which Ruby version we're developing against.
Fix Travis build script.
Fix: rbx build.

On Travis, with rbx-2.2.0, library for `digest/sha2` cannot be found.
I couldn't even install Bundler on my machine with rbx-2.2.0, so it's better to
stick to 2.1.1 for now.
Add badges.

[CI skip]
Read info about an existing repository.
Read package info starting from a repository.
Throw an error if repository doesn't exist.

Meanwhile, I've removed unneeded private writers for entity attrs.
Add #repo? method.
Throw error if package is not found.
Add `Repository#package?` query method.