[#839] Add basic documentation for stats extension
Add note about ckanext.stats.cache_enabled
[#839] Make clear what the different between the stats extension and the page tracking feature are, improve style.
[#839] Add link to stats extension from extensions docs
Conflicts:

	doc/extensions.rst
[#839] Make the docs sound better
[#869] Link to stats docs from extension docs
[#839] Minor edits to stats extension docs

- Edit the opening sentence, it's the stats extension that analyzes the db
  not CKAN (this is picky)
- Also mention that there are tables and graphs!
- Remove weird capitalization in bullet list
- Put built-in page view tracking before ckanext-googleanalytics and tweak
  the link to read a little better
- Add an example of ckan.plugins = stats, also cross-ref to the ckan.plugins
  docs
- Add link to stats page on demo