Add PostgreSQL (basic) add-on

Offer the ability to switch to another PostgreSQL instance, with one of the preinstalled versions (9.2 or 9.3, instead of default 9.1).

I kept this add-on very simple, but we could consider possible improvements:
* try to install requested postgresql on the fly, via `apt-get` (disliked at first glance)
* mark the build as "errored" if requested PostgreSQL version is not available (I like the idea, but it seems that other add-ons - like Firefox - do not fail when add-on configuration is wrong, e.g. trying to install an invalid version). What do you think?

I also have a general question about possible "matrixification" of **add-ons**: Could we consider adding elements like `addons.firefox` or `addons.postgresql` to the travis-core [build matrix](https://github.com/travis-ci/travis-core/blob/master/lib/travis/model/build/matrix.rb#L23)? If not, maybe the `postgresql` feature shouldn't be implemented as an add-on... thoughts?