Use setuptools console_scripts to create gracedb executable

This patch modifies setup.py to provide the gracedb executable via entry_points, rather than a symlink into the bin/ directory. This is more inline with how other projects provide scripts (e.g. pip) and fixes an issue seen on Windows when trying to package ligo-gracedb for conda.

This should have no visible impact on any of the builds or distributions, but makes providing separate python2 and python3 versions of the command-line client trivial, which I will post in a separate merge request after this has been merged.