Improve implementation of error_free_pass_rate
Restructure pass_rate_helper_spec
Display the average number of rebuilds on the project page
Display average elapsed time for builds on project page
Rename PassRateHelper to ProjectStatsHelper
nits
Merge pull request #62 from square/rob/show-avg-attempts

Display average number of rebuilds and average elapsed time on project page

R: jkingdon jackdanger xaviershay