is_iter() in search only works for tuples and lists

[search.is_iter](https://github.com/lugensa/scorched/blob/master/scorched/search.py#L20) returns False for sets (and others). A more appropriate implementation would be:

```
from collections import Iterable

try:
    basestring
except NameError:
    basestring = str

def is_iter(val):
    return not isinstance(val, basestring) and isinstance(val, Iterable)
```
