Add combined disk/volume "devices" to create_config
Add combined disk/volume "devices" to create_config

This closes #60

Introduces another parameter to create_config, "devices", which works as
the API's devices do - you may intermix Disks and Volumes in one list
mapped in device index order.  I left disks and volumes as individual
lists for simplicity, and you may only use them if you don't use
devices.

linode/python-linode-api,jo-tez/python-linode-api