Add a CDB_TableMetadata table and trigger to maintain it updated

The table only contains table name and a last update timestamp.
A trigger, to be attached to tables, takes care of inserting/updating
rows in the table when the data in the subject table is changed.

The metadata table is only writable by the superuser (not the
"db owner" user). The trigger runs with the privileges of the
superuser so can always write in there, whoever performs the
insert/update operation to the trigger-equipped tables.
This is to support inserts into user tables by anonimous users.

In order to upgrade it is important to load the new functions:

 rake cartodb:db:load_functions

And refresh user triggers:

 rake cartodb:db:update_cache_trigger
Guard against infinite loop in CDB_TableMetadata_Trigger
Drop stale records in TableMetadata and improve trigger definition

Trigger definition improvements:
 - also run on DELETE actions
 - drop any previously existing trigger by the same name
merged with develop
Have CDB_TableMetadata_Trigger nofify cdb_tabledata_update <tabname>

Listeners could react by scheduling cache refreshes
Fixes load_functions rake task
Adds rake task to update map calls for every user
Adds some output to update_map_calls task
Copy change: API calls => map views
Merge branch 'table_metadata' into develop
Fixes cdb_checkquota function

Postgres integer was not enough to handle user quotas
Fixes infowindow scroll (in new header image theme) and adds shadow
Shows null value when the title field of the infowindow is empty. Improves infowindow help message
fixed url when tries to upgrade after reach the limits
Implement preliminary data repository for Job models
Placeholder readme for Workman
Implement Job model for Workman service
Implement presenter for Job models
Implement Workman job dispatcher
Implement HTTP API for Workman. To be substituted with a dedicated controller in CartoDB