* 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

add support for pluggable compressor/decompressor

add support for noreply

Empty keys with a prefix are ok (for _multi() mostly).
_multi() was silently converting None key to a string.

Add support for pluggable compressors.

zlib is a fine choice for a default compressor, but in some environments
alternatives (lz4/lzf/snappy) that offer differet tradeoffs for
size/time, may be preferred.