Connection pooling support using 'pg' module's connection pooling 

Added connection pooling support using 'pg' module's in-build connection pooling mechanism

Schema definition contains an additional parameter poolSize that is passed to 'pg' module.
Underscore is fine as dependency of your application code, but I'm trying to reduce number of dependencies in packages to speedup installation process and reduce amount of code per release to deploy. So that dependencies added only when they are really necessary.