Modify project folder struct

	modified:   asyncmemcached/client.py
	modified:   asyncmemcached/connection.py
	modified:   asyncmemcached/error.py
	modified:   asyncmemcached/pool.py
	modified:   setup.py
BUG FIX:release the connection before callback

release the connection before the callback done, will cause reusing the
connection error when callback be set None in last use

	modified:   asyncmemcached/connection.py
	modified:   asyncmemcached/pool.py
	modified:   setup.py