Extension descriptions for easy installation

It'd be really neat if extensions on github (and elsewhere) contained something similar to node's package.json for easy installation.

I envisage this json file would describe:
- Required config options, with defaults which would be added to the .ini
- Post-installation steps, for those extensions that require DB tables (e.g. paster taxonomy init)
- Names of plugins to be added to the ckan.ini file.
- Versions of CKAN that the extension has been tested against.
- Commands to be run to cleanup the extension, remove tables etc.
- Instructions to CKAN on where migrations are held.
- Links to the repo, documentation etc.

Along with requirements in the setup.py this could make the installation of extensions much more straightforward.

The issue names setup.py, CKAN's .ini files, and extension install and cleanup steps, but no source files or tests. Start by mapping how CKAN currently discovers extensions and applies configuration and migrations; done would require an agreed metadata format, installation lifecycle, and concrete implementation tests.

Written by the indexing model from the issue text.