Update sadd to write values as strings

Pyredis writes values as strings for most things, including sadd. This change writes sets as strings:

In [2]: redis.sadd('test_set', *range(3))
Out[2]: 3
In [3]: redis.smembers('test_set')
Out[3]: set(['1', '0', '2'])
Add test for #52
Merge branch 'bwu-patch-1'

* bwu-patch-1:
  Add test for #52
  Update sadd to write values as strings