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`.

I've been doing all this stuff manually for every new extension so this is going to save me some time, will hopefull encourage the general extension writing community to have good practices as well.

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)