Use PostgreSQL as default database

[sheerun]

Reasoning

• After using MySQL for few years, I experienced a lot of quirs.
• PostgreSQL is well-supported by Rails4, most noticeably hstore.
• Heroku's preferred db is postgresql (there are mysql addons though).
• SQLite is easy for set-up but using it on development and postgresql on production can lead to production-only bug.

  Downsides

• Currently there is no good, and free postgresql client for Mac. The ones I am aware of are pgAdmin, Navicat and PG Commander, and pgXplorer.
• You usually have to edit database.yml file before using app (solved by #7)