Rakefile: add default task to list tasks

Signed-off-by: Paul Morgan <jumanjiman@gmail.com>
Rakefile: use bundler to bootstrap clean environment

* use a clean environment when doing dev on this gem
* provide helpful instructions to somebody who is not a ruby expert

Signed-off-by: Paul Morgan <jumanjiman@gmail.com>
gemspec: add rspec dev dependencies

Runtime deps should not be as strict as dev deps since
this gem will be used as a dev dependency for other repos.
spec: add minimal spec_helper

This initial spec_helper:

* framework for requiring gems in every spec test
* show all spec failures in CI console output
* profile slowest examples in CI console output
* protect against any test that may add environment variables
rspec: mock with mocha

* add specific gems as dev dependencies
* configure rspec to use mocha
rspec: preserve env vars between tests

* Provide a shared context in library form
* Include the shared context in rspec configuration

In other projects, I sometimes set environment variables
within rspec examples. This shared context lets me throw
those away after a test runs.