Exch

Added
- offline support
- syncing function
- function to list the currencies available

Changed
- README, better usage examples
- help message, now it shows the current default currencies

Added
- default currency json file

Added
- Feature of setting new default currencies
- Coverage report badge through CodeCov
- CODE_OF_CONDUCT.md
- CONTRIBUTING.md

Changed
- `tox` script to run `pytest` with `coverage`

- Initial relese