v0.4.0

• Added a preamble Slack message when a pipeline starts. Now you'll get notified when a pipeline starts in addition to when it finishes (closes #39)
• Added a SLACK env variable so you can decide if you want to send Slack messages or not
• Added unit tests for the message module
• Documented all env variables in the README
• Fixed a bug where git clone/pull logic was swapped and wasn't returning anything