Improve the ckanext template

Add Travis, Coveralls, PyPI, tests, README, gitignore, LICENSE, etc. to the CKAN extension template that gets rendered when you do `paster --plugin=ckan create -t ckanext`.

Also changed the template to use Jinja2 everywhere.

Example output:

https://github.com/ckan/ckanext-eurovoc

https://pypi.python.org/pypi/ckanext-eurovoc

(Not sure why the rst isn't rendering on pypi though)