Put caller cwd on sys.path for generate metadata

[Hashim1999164]

Fixes #203

The petastorm-generate-metadata.py console script could not locate a project unischema class because installed scripts do not keep the caller's working directory on sys.path. pydoc.locate then fails for modules that live in the project tree.

_main now puts os.getcwd() at the front of sys.path before resolving --unischema_class. A unit test covers that cwd wins over an install bin path.

Test plan

1. Run test_ensure_cwd_on_sys_path_puts_cwd_first
2. From a project directory, run petastorm-generate-metadata.py with --unischema_class pointing at a local module and confirm it loads

[CLAassistant]

All committers have signed the CLA.