Add ability to import and dump databases using the Postgresql_db module

Tested using the following playbook and an existing database dump:

    - hosts: dbserver
      tasks:
        - name: transfer schema
          copy: src=/foo/bar.sql 
                dest=~/import.sql

        - name: test import
          postgresql_db: name=test1
                         login_user=dbuser
                         login_password=dbuser
                         state=import
                         target='~/import.sql'

        - name: test dump
          postgresql_db: name=test1
                         login_user=dbuser
                         login_password=dbuser
                         state=dump
                         target='~/dump.sql'

        - name: test reimport
          postgresql_db: name=test2
                         login_user=dbuser
                         login_password=dbuser
                         state=import
                         target='~/dump.sql'