Piper

Changed
- Addresses [218](https://github.com/databio/pypiper/issues/218)

Changed
- remove pipestat_project_name from PipelineManager parameters
- refactor pipestat_sample_name to pipestat_record_identifier in PipelineManager parameters
- update requirements for pipestat 0.9.0, ubiquerg 0.8.0, and yacman 0.9.3
- set `force_overwrite` to default to true, Issue 209

Changed
- refactor for pipestat v0.6.0 release
- drop python 2.7
- updated requirements
- changed message_raw to be a value_dict when reporting to conform to pipestat
- Fixed
- fixed 196 and 197
- Added
- added `force_overwrite` to `report_result` and `report_object`
- added pipestat_pipeline_type, defaulting to sample-level

Fixed
- fixed self.new_start overriding checkpoints.

Fixed
- added _safe_write_to_file back into pypiper for Pepatac backwards compatibility

Added

- [pipestat](http://pipestat.databio.org/en/latest/) support