Getgauge

- Adding python3 version compatibility check.
- Updating step implementation parsing logic.
- Adding `multiple implementations found for a step` error type to validation errors.

Enhancements
- Adding requirements.txt file with `getgauge` dependency while initializing a gauge project

Bug Fixes
- Using `PYTHONUNBUFFERED` Instead of -u flag for unbuffered output.

> Removed `python 2.x` support

Enhancements
- Improved step implementation loader.

New Features
- Python 3 support

Enhancements
- Enhanced and optimized step implementation scanning logic.
- Taking screenshot when the value of `screenshot_on_failure` is set to true.
- Some refactoring around `data_stores` and `Table`.

New Features
- Debugging support
- Step aliases support
- Custom screenshot hook