Implement multiple backend option

[lucasrcezimbra]

https://github.com/HBN3tw0rk/dcache/tree/master#multiple-backends

Ideia:

from dcache import dcache
from dcache.backends import InMemoryBackend, RedisBackend

@dcache(multiple=[
    InMemoryBackend(),
    RedisBackend(host='localhost', port=6379, db=0),
])
def slow_function(n):
        return n ** 1000

1. search on the in-memory cache;
2. if exists, return, if not, search on Redis;
3. • if exists on Redis, save in memory and return;
4. • if not, exists on Redis, run the slow_function, save on Redis, save in-memory and return;

• doesn't run if already returned