Add IndexExists() support to PostgreSQL dialect.

Before, calling `CreateTableIfNotExists()` on a struct with indexed fields resulted in an error if the table already existed, e.g.:

```
2013/03/13 01:39:24 pq: S:"ERROR" F:"index.c" C:"42P07" L:"774" M:"relation \"email\" already exists" R:"index_create"
```

Now Postgres dialect can check to see if an index already exists.