Implement Statistics api

Fixes #294

:dancer: 

### Todo

- [x] Implement get contributors list
- [x] Implement get commit activity data
- [x] Implement get number of additions and deletions per week
- [x] Implement get weekly commit count 
- [x] Implement get commits per hour
- [x] Handle 202, 200 for queued jobs
- [x] Tests