Adds Discord Notifications

[night]

This pull request adds the ability to send build status updates to Discord via webhooks.

The configuration format looks like the following:

notifications:
  discord:
    channels:
      - [webhook_id]:[webhook_token]
      - [webhook_id_2]:[webhook_token_2]
    pull_request_template:
      - 'Build [#%{build_number}](%{build_url}) ([%{commit}](%{compare_url})) of %{repository}@%{branch} in PR [#%{pull_request_number}](%{pull_request_url}) by %{author} %{result} in %{duration}'
    branch_template:
      - 'Build [#%{build_number}](%{build_url}) ([%{commit}](%{compare_url})) of %{repository}@%{branch} by %{author} %{result} in %{duration}'

Related PRs:
travis-ci/travis-hub#84
travis-ci/travis-tasks#71

[Arzte]

This pr's build was broken by a dependency error.

[night]

@TheDoctorsLife you don't say.. check #565 for the unmerged fix for that

[BanzaiMan]

This PR is no longer necessary for adding notifications.