Fixes PostgresSQL connection problems.

This commit fixes the following connection problems:
* Pools should now properly release themselves always
* You can now set a "TestOnBorrow"-like functionality with
  config.pool.reapInterval).
* Heroku should be able to properly send the correct DB credentials.
Postgres single connections will no longer leak

Some users were experiencing problems with Postgres connections leaking when
they didn't enable pooling. This commit should fix the problem.

- In prototype.disconnect() we now properly call this.client.end with
  this.client.end.bind(this.client) (as shown in the node-postgres wiki).
- We no longer check for readyForQuery on this.client as that value may not
  always be populated by node-postgres (possibly a bug on node-postgres).

This should close https://github.com/sequelize/sequelize/issues/859
Merge pull request #860 from durango/postgres-connection

Fix: Postgres connection issues