Add class BuildServer.

This class has the following methods:

  create - build new server.
  commands - execute commands on the server.
  delete - delete the server.

Method 'create' has the folloving options :
  name='unnamed'
  flavor='512'
  image='cirros-0.3.1-x86_64-uec'
  pkey='last_key'

Method 'commands' has the folloving opttion:
  command='echo "Hello, world"'