Agents setup.py fixes

[scragraham]

• resolves #17
• resolves #18
• Cleanup of script description to allow upload to pypi
• replace repo url with appscale's

[scragraham]

Before fix:

$ twine check dist/appscale-agents-3.8.0.tar.gz
Checking distribution dist/appscale-agents-3.8.0.tar.gz: warning: `long_description_content_type` missing.  defaulting to `text/x-rst`.
Failed
The project's long_description has invalid markup which will not be rendered on PyPI. The following syntax errors were detected:
line 2: Warning: Title underline too short.

AppScale Agents
--------------

After fix:

$ twine check dist/appscale-agents-3.8.0.tar.gz
Checking distribution dist/appscale-agents-3.8.0.tar.gz: warning: `long_description_content_type` missing.  defaulting to `text/x-rst`.
Passed