Completely promisify pg driver

lib/adapters/Pg.js

@@ -32,12 +32,14 @@ class Pg extends Adapter {
 		}
 
 		if (connectionString !== '') {
-			instance = new pg.Client(connectionString);
-			instance.connect(err => {
+			let conn = new pg.Client(connectionString);
+			conn.connect(err => {
 				if (err) {
 					throw new Error(err);
 				}
 			});
+
+			instance = Promise.resolve(conn);
 		}
 
 		super(instance);
@@ -67,32 +69,25 @@ class Pg extends Adapter {
 	 *
 	 * @param {String} sql - The sql with placeholders
 	 * @param {Array} params - The values to insert into the query
-	 * @param {Function} [callback] - Callback to run when a response is recieved
 	 * @return {void|Promise} - Returns a promise if no callback is provided
 	 */
-	execute (/* sql, params, callback */) {
-		let args = getArgs('sql:string, [params]:array, [callback]:function', arguments);
+	execute (sql, params) {
 
 		// Replace question marks with numbered placeholders, because this adapter is different...
 		let count = 0;
-		args.sql = args.sql.replace(/\?/g, () => {
+		sql = sql.replace(/\?/g, () => {
 			count++;
 			return `$${count}`;
 		});
 
-		if (!args.callback) {
+		return this.instance.then(conn => {
 			return new Promise((resolve, reject) => {
-				this.instance.query(args.sql, args.params, (err, result) =>
+                conn.query(sql, params, (err, result) =>
 					(err)
 						? reject(err)
 						: resolve(this.transformResult(result))
 				);
 			});
-		}
-
-		return this.instance.query(args.sql, args.params, (err, origResult) => {
-			let result = this.transformResult(origResult);
-			return args.callback(err, result);
 		});
 	}
 }