differentiate boot drives and data drives

boot drives have an image_type attribute data drives do not
release ip bug fix

check for null before parsing the json object
os icon and ip bug fix

fogbugz 2899 - os icon not displayed for servers.
api 2.0 changed w/regards server ip details so implementation updated
to parse the new object
attach volume big fix FB2017

cloudsigma now only allow up to 4 units per controller
Merge pull request #2 from daniellemayne/2013.02

bug fixes and code tidy up