add a detach volume function

add a detach volume function, require  serverId, attacmentId, delete
this attachment, add a function 'getAttachments()' in class
Volume,return an attachment object contains
device,hostname,id,server_id,volume_id
change some annotation and variable name

change some annotation and variable name
change some annotation and variable name

change some annotation and variable name
Merge pull request #37 from octupszhang/master

add a detaching volume funtion