Add the `adder` tool gem.

This tool will be the example for the first exposure to the BDD / TDD
process. It is setup with some basic structure to facilitate the process
without requiring lots of extra custom configurations.

RSpec is being used for the testing tool. All TDD / BDD can be done with
most modern testing tools. BDD does not require a tool such as Cucumber,
but Cucumber, and other similar tools, can help facility communication
with end users.

For this tool, all testing will be done in RSpec. A sample `.rspec` file
is included to show how some custom settings can be added. Additionally,
a `spec_helper` is updated to allow for some BDD type specific
enhancements; such as filtering out any works in progress.
Start with a high level feature specification.

With BDD, focus is generally on a top-down approach. It also focuses on
how end-users interact with the program. It is not concerned necessarily
with implementation details.

Using this approach, the feature specs are based around user / job
stories. They view the program as more of a black box. Due to this, most
tests tend to be full integration tests where no mocks or stubs are
used. Instead, the focus is solely on environment state changes.

Since some features can a involved and require lots of new code, they
are generally marked as `wip` for "work in progress". This allows the
incomplete feature to be excluded from the main test run, while it is
implemented.