fix postgresql:link

use $3 in there, since it actually points to a postgresql instance name
Merge pull request #27 from piranha/patch-1

fix postgresql:link
Added database dump
Added database restore
README.md updated
Check if pg_dump/psql are installed
Fixed two small typos
Removed unecessary 'DB_IMAGE' variables
Refactored PostgreSQL tools check (@Zenedith tip)
Refactored PostgreSQL container check (@Zenedith)