Revamp 'postgresql': multi-versions and PostGIS

- Use PDPG apt repository
- Install multiple instances (9.1, 9.2 and 9.3)
- Preinstall PostGIS 2.1 for all instances
- Only supports Ubuntu 12.04LTS (all outdated and unused recipes
  original Opscode cookbook have been removed. e.g. Redhat/RPM flavors)
- Customize /etc/init.d/postgresql to be backward compatible (default
  postgresql version is used if not specified), and to prevent to start
  more than one instance at the same time
- Database copy to RAMFS storage is now explicitly handled by chef provision
  (instead of idempotent copy in /etc/init.d/postgresql script)
- CI-Specific tunings are still there (and parametrized, if not already)
  see original commits 29f200f5bc and f34cf19a68.
- Clean and fix handling of /etc/sysctl.conf and /etc/sysctl.d/...
- Refactor 'sysctl' cookbook to dynamically compute System V shared
  memory depending on 'postgresql' configuration (`max_connection`)
- Add a parameter in 'ramfs' cookbook to adjust disk size (default is
  still 512M)
- Set max_connections = 255 as default (No more needed to override this
  attribute in travis-images standard template)

The resulting cookbook is simply organized (less recipes) and strictly
focused on Continuation Integration context.
The relashionship with very old forked version (0.11) of original Opscode
cookook doesn't make sense any more. For these reasons I bump this
cookbook to version 2.0.0, in order to clearly mark this step, but not
to express maturity level.
[postgresql] Fix bug in RAMFS management

In order to keep it simple, data for all instances are always copied.
We could try to improve this to spare some RAMFS space, but it is
quite more tricky to manage the different use cases in a robust manner.
Merge pull request #221 from gildegoma/multi-pg-and-postgis

Revamp 'postgresql': Add support of PostgreSQL 9.2 and 9.3, and also PostGIS 2.1