Mock npm adduser server response, fixing #5

put /-/user/:name first
if return 201 adduser ok,
else if return 409, means user exist

then get /-/user/:name with header authorization: 'Basic ZXhpc3EWdf=='
if check ok, return 200 and account info. then put/-/user/:name/-rev/:rev
if check not ok, return 401, adduser error.

adduser the same account:
It will post /_session to get authSession.
Merge pull request #9 from fengmk2/adduser-mock

Mock npm adduser server response, fixing #5