Use environment and pgpass to connect to PostgreSQL

[Natureshadow]

Closes #384.

Instead of building a command-line that might be subject to
a row of issues (the command-line used before carried the
database password in plaintext, making it available to everyone
with read access to the system, we now build a copy of the
environment with all the libpq variables that are needed prefilled.
The password is written into a .pgpass file as suggested by the
PostgreSQL docs.

This also makes the call more compatible with various setups. For
instance, a hostname is not mandatory — pg_dump can connect
through the UNIX socket in /var/run as well.

In order to make all that re-usable and easy to read and
understand, I decided to pack the environment generation
and .pgpass file handling in a context manager. Someone
else (or me, later on, if I get bored) might want to copy it for
MySQL and MongoDB.