Rough implementation of PostgreSQL JSON support

This is a rough implementation of the JSON datatype in Postgres 9.3 but can easily be extended for JSONB in 9.4.

The query interface needs to be discussed further (see #1016). In this implementation both the `where` and `attributes` options work for finders. Currently, querying JSON data-types uses the same query language as Postgres without providing any abstractions.

Things yet to do:
- Discuss API further and update implementation accordingly
- Update documentation
- Implement JSONB from PostgreSQL 9.4

Here a simple example of querying:
```javascript
Content.find({ where: "metadata->>'language' = 'English'" })
  .then(function(content) {
    console.log('Result:', content.dataValues);
  })
```

A more thorough example can be found under [examples/postgresql-json-support](https://github.com/sveinnfannar/sequelize/blob/feature/postgresql-json-support/examples/postgres-json-datatype/app.js).