[#781] Begin improving IFacets docs

Better docstrings and an example extension for IFacets. Both still need
some work..
[#1350] resource format change
[#1386] Add factories for Dataset and Resources
[#1386] Allow to define auth user when using factories

When creating an instance of the factory, users can provide an 'auth_user'
keyword argument with the name of the user that should perform the action.
If they don't provide the 'auth_user' argument the site user will be used
(this user is a sysadmin). If the action should be performed anonymously
(ie no logged in user), auth_user=None should be provided.

Note that the keyword argument could not be named 'user', as some action
functions use this same parameter.
[#1386] PEP8 fixes
[#1364] Change labelling on homepage search area
[#1332] Makes markdown link a popover with context of CKAN flavoured markdown
[#1430] show package/resource revision_ids, disallow setting them
[#1330] Get user ids from a list of ids or names
[#1330] Filter activity from specified users
[#1330] Update documentation
[#1330] Use actual value of site_id for default
[#1462] Add guidelines for reviewing pull requests
Merge branch 'master' of github.com:okfn/ckan into 1462-reviewing-guidelines
[#1330] Fix test failures
pep8 fix for test failure
Update action function and documentation

* Split the helper function into two functions.
* Add better docstrings.
* Improve the documentation and explain about site user.
Fix tiny error.
[#790] Tutorial for custom fields in packages
[#790] add custom fields tutorial to extensions index