Skip issuing a PR if one already exists for -b option

[joestump]

• This checks for an existing PR for head into base. If a PR exists, we skip issuing the PR. Cleans up the code to comply with Rubocop.
• Adds full test coverage for the manage_pr method.
• Adds a default task for rake.
• Switches output over to $stdout and $stderr.

[bastelfreak]

Hi @joestump, thanks for the PR. @ekohl can you please also review this?

[joestump]

I'll add tests once we figure out what we want to do with #149. There are tests for the entire manage_pr method in that PR (even if we just merge those for now).

[joestump]

@bastelfreak @ekohl this is ready for a final review.