Merge pull request #865 from hasegeli/master

Postgresql module is often extended by other shared modules. Shared modules should not use resource-like class definitions to let the callers use them when necessary. It is still a reasonable behaviour for the shared module to set max_connections. Making it optional would allow this to happen.

Removing the default value "localhost" should not break anything as it is already the default of PostgreSQL and on every distribution I know of.