Empty sequences yield error

The filter lacks checks for empty strings or lists.

Simple example:

```
from bloom_filter import BloomFilter
bloom=BloomFilter()
'' in bloom
```
```
.../bloom_filter/bloom_filter.py in get_bitno_lin_comb(bloom_filter, key)
    478             int_list.append(remainder)
    479             temp = quotient
--> 480     elif hasattr(key[0], '__divmod__'):
```
