Implement pttl
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'])
Stringify values on hmset
Add test for stringly-typed hmset values
Merge branch 'danielsoneg-hmset-strings'

* danielsoneg-hmset-strings:
  Add test for stringly-typed hmset values
  Stringify values on hmset
Use builtin python +inf/-inf values for zrange

Simplifies the code a bit.
Merge branch 'oyiptong-master'

* oyiptong-master:
  Use builtin python +inf/-inf values for zrange
  add +inf and -inf to sorted set min/max queries
Add test for #52
Merge branch 'bwu-patch-1'

* bwu-patch-1:
  Add test for #52
  Update sadd to write values as strings
Merge branch 'clee704-pttl'

* clee704-pttl:
  Implement pttl