Use slug to identify images

DigitalOcean’s API changed and the images endpoint does no longer
include the distribution (“7.0 x64” instead of “Debian 7.0 x64”). Use
the slug instead which is unique and human readable. Change the README to reflect that.
Add digitalocan-list command

The command lets the user list available images, regions and sizes.
Better wording for the list command
Merge pull request #165 from pascalj/master

Use the slug to identify images