Support postgreSQL 'postgresql://' scheme.

Thank you for make useful library.
I have used sequelize(1.7.0) with postgreSQL.
And I realized that sequelize didn't support `postgresql://` schema.

This is from [PostgreSQL Connection String Spec.](http://www.postgresql.org/docs/9.2/static/libpq-connect.html#LIBPQ-CONNSTRING)
> The URI scheme designator can be either postgresql:// or postgres://.

The following code produces an error.

```
var sequelize = new Sequelize('postgresql://localhost/sequelize_test')
```

And error messages.

```
  1) [POSTGRES] Sequelize constructor should work with connection strings (2):
     TypeError: Cannot set property 'dialect' of undefined
    at new module.exports.Sequelize (/Users/sangpire/play/sequelize/lib/sequelize.js:101:29)
    at Context.<anonymous> (/Users/sangpire/play/sequelize/test/sequelize.test.js:61:25)
    at Test.Runnable.run (/Users/sangpire/play/sequelize/node_modules/mocha/lib/runnable.js:221:32)
    at Runner.runTest (/Users/sangpire/play/sequelize/node_modules/mocha/lib/runner.js:374:10)
    at /Users/sangpire/play/sequelize/node_modules/mocha/lib/runner.js:452:12
    at next (/Users/sangpire/play/sequelize/node_modules/mocha/lib/runner.js:299:14)
    at /Users/sangpire/play/sequelize/node_modules/mocha/lib/runner.js:309:7
    at next (/Users/sangpire/play/sequelize/node_modules/mocha/lib/runner.js:247:23)
    at Object._onImmediate (/Users/sangpire/play/sequelize/node_modules/mocha/lib/runner.js:276:5)
    at processImmediate [as _immediateCallback] (timers.js:330:15)
```

Thank you.