List PostgreSQL library as requirement

[jwnx]

The psycopg2 package listed in requirements.txt needs libpq-dev (and similar packages) to be installed. It would be nice to list libpq-dev as a development dependency.

For Ubuntu

sudo apt-get install libpq-dev

For CentOS

sudo yum install postgresql-devel

For OSX

brew install postgresql

(you get the idea)