Update the development guide with an improved package management script and newly added packages (#229)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/229

## This diff
 - Added `pytest` and other packages to enable running python tests with `pytest` (`pytest.ini` will be added separately)
 - Added all required python packages to `requirements.txt`
 - Moved `install_requirements.sh` to `executorch` root
 - Consolidated py deps installation by simply running `bash ./install_requirements.sh`
 - Updated `examples/README.md` to reflect the package installation changes
 - Revisited `00_setting_up_executorch.md` with simplified workflow to set up executorch dev

## Next
  1. Add a global config for pytest, i.e. `pytest.init`
  2. Cover all successfully runnable python tests in OSS CI

NOTE: Not all python tests could run successfully, so we will need to resolve issues one-by-one. Full list of python tests in `executorch` can be found here: P821423083

Reviewed By: cccclai

Differential Revision: D49037160

fbshipit-source-id: bc755072b43b04c38a6109e22db354ae6a4aefd2