Add support to select fields with double quotes

Hi,
this pull request solve the issue "The gorm way to select a column #94".

Now we can select fields quoting them with dobule quotes. Useful when you got a field named in the same way of a SQL reserved word.

Now we can generate query like
```sql
SELECT "from" FROM bookmarks WHERE hpid = 3
```
In that way
```go
// code ...
    db.Model(UserBookmark{}).Where(&UserBookmark{Hpid: post.Hpid}).Pluck("\"from\"", &users)
}
```