Add gets method and simplify connection estabilish

- simplify connection estabilish procedure, as tornado.iostream
  permit write before connection estabilished, I removed the
  callback and connection timeout procedure in _get_server()
- add `gets(self, keys, callback, failcallback)` method, receive
  a list of keys as parameter and return a dictionary of results
  result = {key1:value1, key2:value2...}, only return the got
  keys from memcached
- some debug info, might be cleanup later

Signed-off-by: Wang Xu <gnawux@gmail.com>