Have create_publicuser rake task not complain if user already exists

This commit uses the PostgreSQL 9.0+ "DO" construct to only create
the users when they don't exist already. We require 9.1+ already
so no problem about that.
fixed wizard regeneration on first table fetch CDB-1454
fixed spec CDB-1454
fixed choropleth when there are no columns CDB-1456
Added task to see modified layers in a period
fixed category list when values are null CDB-1458
Added rescue to layers rake
Added visualization type to layers rake
Merge remote-tracking branch 'origin/CDB-1458' into hotfix/20140115
Merge remote-tracking branch 'origin/CDB-1454' into hotfix/20140115
CDB-1455 #resolves Only show one warning message when the user is close to the size & table limit
Note about missing 2.7.0 release in NEWS.md
Added release dates in the NEWS file (#314)
Merge remote-tracking branch 'origin/CDB-1462' into release/2.8.1
Updated release notes (v2.8.1)
Merge remote-tracking branch 'origin/CDB-1455' into release/2.8.1
Update cartodb.js
fixed saving previous wrong state CDB-1496
update table after georeference CDB-1497
Added varnish PPA to README.md