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.