Extensions registry awareness in ckanext template

[jqnatividad]

It'd be nice to integrate some awareness of http://github.com/ckan/extensions.ckan.org (which generates http://extensions.ckan.org) when creating new extensions:

• check for extension name collisions
• automatically generate extension registry metadata (http://extensions.ckan.org/add/)
• subtle way to do extension discovery (e.g. somebody already built an LDAP extension)
• optionally push WIP extensions to the registry. Should be a good way for extension developers to find external collaborators (maybe in addition to the featured tag, we can even have a "need collaborator" tag)
• possibly create a mechanism to send extension metadata PRs to extension registry when creating extension releases (adding ability to have extension metadata over several releases in the registry, not just the latest one)

The ckanext template was recently improved and it bakes in some extension development best practices.

Registering/Pre-registering could be another best practice that can be baked in:

• it makes sure the registry stays current
• additional glue for CKAN developer community to track WIP extension development efforts, allowing CKAN users to track the growth of the CKAN extension ecosystem
• Cleaner/richer data in the registry can be leveraged for an apt-get like mechanism for installing extensions in the future
• Create automatic CKAN developer ecosystem content (e.g. latest extensions released/started/updated this week/month, links to sample implementations w/ screenshots, etc.)

This doesn't remove the need for registry curation, but will make it more organic/automatic.

[jqnatividad]

Also see #113