Pyranges

- Added option to read_gtf to rename columns that have reserved names in pyranges https://github.com/pyranges/pyranges/issues/341

- Added new documentation centralized at readthedocs (including tutorial, how-to-pages, dev guide)
- Shrunk README accordingly
- Added new tests based on doctests
- Fixed future warning https://github.com/pyranges/pyranges/issues/349

- Fix join left category error 342 for real

- Fix join left category error 342

- Fix tostring regression

- add preserve_order to join (aallahyar)