Adds Octokit gem to access GitHub API.
Adds dotenv gem.

This gem allow to load environment variables from `.env`.
URL: https://github.com/bkeepers/dotenv
Adds gitignore file and removes unwanted files.
Configures RSpec's spec helper.
Adds Github service.

The Github service is just an utility class (for now)
to make the interaction with the Github API easier.

It encapsulates the logic to create webhooks and
finally delegates the task to a Github client.
We are using Octokit as the Github client.
Adds .env-template

The .env-template file is supposed to be used a guideline
for developer to generate their own .env file.
Adds Rake task to generate a github webhook.
Adds GithubConfig module

GithubConfig module it ment to be included in clases that
want to access the application github's configuration.
It forwardes method to the Rails.application.config.github
object.
Extract assembly of repo full name to private method.
Configure RSpec to use documentation formatter.
Removes the use of 'should' in spec it description.
Adds de possibility to edit a webhook URL.
Adds Github#fetch_webhook_id
Changes the Rake task github:edit_webhook_url.

The task does not require the webhook id anymore because
it uses the github service to fetch it.
Describes the behavior of Github.new.
Adds PullRequest model and its factory.

It alse add its corresponding migration and schema update.
Adds database_cleaner gem.

Configures RSpec to use database cleaner.
Adds a PullRequestEventManager class.

The PullRequestEventManager is responsible for handling
the payload of pull request webhook sent by Github. It
dispatch the payload to a class that knows how to handle
the specific action of the pull request event.

Possible actions are "assigned", "unassigned", "labeled",
"unlabeled", "opened", "closed".

The only supported action (for now) is opened and the class
that handles that payload is
PullRequestEventManager::OpenedPullRequest. It creates a new
pull request object. A spec is also added for this class.

Github payload for testing are loading using JSON fixtures.
A module has been configured in RSpec to make JSON fixtures
loading easier.
Add ngrok to .gitignore.
Makes valid_action? & supported_action? methods public.

Now users of the PullRequestEventManager can avoid an exception
when calling the handle_pull_request_event method.