Add queue completion estimate in output of queue health
Add estimation for completion of current job
Add more information in output of health command

- Oldest and Most recently executed Jobs
- Oldest and Most recently cancelled Jobs
- Oldest and Most recently executed Errors
Extract stats functions in a seperate module
Refactor function names in stats
Fix problem with inclusion of current job estimate
Beautify code for pylint
Fix tests to account for precision in date time
Update changelog
Bump version number
Add another algorithm to estimate queue completion
Fix management command and its test

Management command for health takes argument
for estimation algorithm, the default being
the one for rough estimate.
Add aggregate for executed, unexecuted, and cancelled jobs

Queue health shows details (name, and count) of jobs that
have been executed, cancelled, and are unexecuted.
Merge pull request #13 from suhridsatyal/develop

Improved health monitoring.