remove branch parameter to install_github
Deprecate and remove pull parameter to install_github

- New exported function github_pull() to be used as value for ref parameter
- Implemented using a simple S3 dispatch mechanism
- Tests with improved mocking
    - new function with_mock
- Tests for warning when using deprecated parameters branch and pull
update NEWS
don't use with(), fix indentation
clean up
refactor: inline function, rename variables; this affects an existing test
document S3 generic
rename class pull to github_pull, inline function github_pull_info
Merge pull request #509 from krlmlr/509-install_github-s3-ref

Clean up install_github parameters pull and branch
Prompt to install build tools when needed.
Closes #488