Allow to specify PostgreSQL connection params as database url

Database url format is a common way of representing connection params
in PaaS computing services like Heroku where service configuration is exposed
via OS environment variables.

The database url can be specified directly or point to env $VARIABLE_NAME.
Update docs to indicate usage of database connection url
Synchronize PostgreSQL db structure upon connection
Merge pull request #24 from nigma/pg-db-url

Allow to specify PostgreSQL connection params as database url