Port memcache to Python 3

* travis: make python 3 tests voting (cannot fail anymore)
* setup.py: add Python 3 classifiers
* Encode unicode key to UTF-8: add _encode_key() method
* Add _encode_cmd() helper method to format a memcache command as a byte
  string (bytes%args will only be supported in Python 3.5)
* Rewrite _map_and_prefix_keys() code converting keys
* _val_to_store_info() now accepts Unicode: Unicode is encoded to UTF-8
* _set('cas') doesn't call _val_to_store_info() anymore when it's not
  needed: begin by checking if the key is in the cas_ids dictionary
* Process server reply as bytes
* _recv_value() now clears the _FLAG_COMPRESSED flag after decompressing
  to simplify the code
* On Python 3, _recv_value() now decodes byte strings from UTF-8
* Simplify check_key(), _encode_key() now encodes Unicode to UTF-8
* Replace u'...' with six.u('...') in tests for Python 3.2

Author: vstinner

.travis.yml

@@ -6,11 +6,6 @@ python:
   - 3.3
   - 3.4
   - pypy
-matrix:
-  allow_failures:
-    - python: 3.2
-    - python: 3.3
-    - python: 3.4
 services:
   - memcached
 install: python setup.py install