Import the blog feature from ckanext-opendata

Import the blog plugin from github.com/Softhouse/ckanext-opendata.

Minimal changes have been made to the plugin:

- Make it work as a standalone plugin (not requiring other plugins from
  ckanext-opendata to be activated at the same time)

- Make it work with CKAN 2.2

- Integrate it into ckanext-sweden as ckanext.sweden.blog
Lint the blog plugin

PyLint, PEP-8 etc. fixes to the blog plugin. Lots of style fixes,
removing unused imports, and bug fixes.