Add entry-point in tools for pyethtool

This removes the `main()` method from the library
part of `pyethereum/pyethtool.py` and
moves it into `tools/pyethtool_cli.py`. It also adds a console_script entry in
`setup.py`, so after installation, the pyethtool-CLI part becomes available as `pyethtool`.

(If you're using buildout, this will simply be `bin/pyethtool`).

Closes #59