add status images support, just like travis
correct status image url
correct project status image url
Fix rbenv ENV propagation issue
Update changelog
Clean up basic auth.

Delete hardcoded admin/secret default.

Only apply basic auth if user and password are configured.
Don't delete the gem lockfile.
Fix trim_branches being in the middle of auth section.
Allow configuring checkout command (#168, etc.)
Display build duration for completed builds; display start time for in-progress builds
Reimplement DateTime#to_time since we do not always have activesupport around
Generate a build with known times for the test
Change ADMIN_USER to ADMIN_USERNAME for consistency with username configuration setting.
Use username and password in variable names also
Accept old environment variables for compatibility
Refer to the variables correctly in documentation
Password environment variable was the same, use a simpler approach
Documentation for building private repositories in general and on Heroku
Try a current version of chronic_duration
Make datetime to time conversion work in more cases