Fix string escape with postgresql on raw SQL queries

In raw SQL queries (sequelize.query) the string escape is \ but for postgre it's ' (http://www.postgresql.org/docs/8.2/static/sql-syntax-lexical.html#SQL-SYNTAX-STRINGS).

**example:**
```javascript
var sql = 'UPDATE "test" SET "value"= ? WHERE id = 1';
sequelize.query(sql, {}, null, "isn't a good string escape" );
```

give into sql:
```sql
UPDATE "test" SET "value"= 'isn\'t a good string escape' WHERE id = 1
```
this request emit an error on postgre.

the request should be:
```sql
UPDATE "test" SET "value"= 'isn''t a good string escape' WHERE id = 1
```