Make postresql stuff optional

[techtonik]

There is a decline in amount of contributors to this project. I can't say what are the main reasons exactly (#902), but for me (I am on Windows machine with 3Gb of system memory) Vagrant box is a very slow option and setting up PostgreSQL is a major overkill - for example pip doesn't install bindings.

So, is it possible to run the site on different backend for development? I think that Django should be independent of database.

[berkerpeksag]

Two things:

• The jobs app doesn't work with the sqlite backend (it's uses some PostgreSQL specific features)
• We want to use the same database configuration in development to avoid possible regressions