Add swift block backend support

I add swift( http://swift.openstack.org ) block backend support,but right now  swift user authentication information must set in common/block-backend-swift.c:

#define DEFAULT_TENANTNAME "myproject"
#define DEAFULT_USERNAME "renwofei"
#define DEFAULT_PASSWORD "123123"

Maybe in the future with seafile user bind. 

You must set seafile-data/seafile.conf like this:

[block_backend]
name=swift
scheme=http
host=172.19.69.170
port=5000
api_version=v2.0
auth_url_ext=tokens
container=swiftbox