set_index added

Added `set_index`. This function will take care that there is no other index on given field.

No need to write:

```
obj.remove_index('field_int', 1)
obj.remove_index('field_int', 2)
obj.add_index('field_int', 3)
```

Now just:

```
obj.set_index('field_int', 3)
```