Add Client Pool

# Usage

```python
from toredis.pool import ClientPool

from tornado.gen import engine as gen_engine
from tornado.gen import Task as gen_Task

from tornado.ioloop import IOLoop

@gen_engine
def info():
    redis_pool = ClientPool(pool_size=20, host='127.0.0.1', port=6379)
    response = yield gen_Task(redis_pool.client.info)

    print response
    IOLoop.instance().stop()

info()
IOLoop.instance().start()