[#746] Extend Pylons installer to tweak deployment.ini_tmpl

With the new 'paste.app_install' entry point on setup.py, we can define
a custom "installer" that basically allows us to define new variables to
be used on the deployment.ini_tmpl file. This is useful eg for defining
the ckan version and provide a link to the relevant documentation for
the configuration options.
[#746] Document datastore db url config options

And include them from the datastore setup documentation to avoid
duplication.
[#746] Update configuration.rst

* Document ckan.email_notifications_since
* Remove duplicate search.facet.limits
* Minor formatting tweaks
[#746] Document file store options in configuration.rst

TODO: Expand them
[#746] Update deployment.ini_tmpl

Remove legacy  and unused options, add new ones. Change the structure to
follow more closely the documentation on configuration.rst. Remove all inline
comments to avoid having documentation in more than one place.
[#746] No : at end of sentence.
[#746] Edit a docstring in deployment.ini_tmpl

Minor edits
[#746] Minor edits to configuration docs
Merge branch 'master' of github.com:okfn/ckan into 746-update-deployment.ini_tmpl

Conflicts:
	ckan/config/deployment.ini_tmpl
	doc/datastore-setup.rst
Merge branch '746-update-deployment.ini_tmpl'