Move session, login and logout methods from Resource to Account

It's a bit awkward to have the code:

  db = cloudant.Database(url)
  doc = db.document(docid)
  doc.login('username', 'password')

Since these methods are not related to document nor database resources.
Account as holder of server root endpoint method is more suitable place
for them.
Merge pull request #35 from kxepal/normalize-access-to-session

Move session, login and logout methods from Resource to Account