Add default value for connection host.

Native binding connection tests will fail if the PGHOST environment variable is not set.

There already exists several defaul values for user, password and port. It would make sense to have a default host setting as well - localhost appears to be a reasonable default.

lib/defaults.js

@@ -1,4 +1,7 @@
 module.exports = {
+  // database host defaults to localhost
+  host: 'localhost',
+
   //database user's name
   user: process.env.USER,