Get And Touch implementation

[svanellewee]

Good day!
Added Get And Touch Implementation with simple test case.
As per:
https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L348-L384

Questions:

• Currently my change assumes latest memcache. Must I add some version check/bool config switch to either enable the gat calls or throw NotImplemented when underlying memcache is too old? (Please specify a preference)
• are the test cases I added sufficient? I also did semi-manual testing using the following:

from __future__ import print_function
from memcache import Client
import random
import time
client = Client(["127.0.0.1:11211"], debug=1, cache_cas=True)
ttl = 1
key = "greeting_{}".format(random.randint(10,1000))
print(client.set(key, "hello", ttl))
print(20 * "-")
reattempts = 10
for i in range(reattempts):
    result = client.gets(key)
    result = result if result is not None else "Nothing!" * 3
    print("GETS: result === {}".format(result))
    time.sleep(0.5)
    for cas_attempt in range(3):
        cas_result = client.cas(key, "hello", ttl)
        if not cas_result:
            break
        print("GETS: cas {}".format(cas_attempt))
key = "greeting_{}".format(random.randint(10,1000))
print(client.set(key, "hello", ttl))
print(20 * "-")
for i in range(reattempts):
    result = client.gats(key, 2)
    result = result if result is not None else "Nothing!" * 3
    print("GATS: result === {}".format(result))
    time.sleep(0.5)
    for cas_attempt in range(3):
        cas_result = client.cas(key, "hello", ttl)
        if not cas_result:
            break

[svanellewee]

Would it be okay for me to update the .travis.yml to use the latest memcached ?