Use a comma separated list of projects in project tag

Hi I often use the tag 'Project' in accounts which hold several projects. And now there's one of these account that stores a machine wich belongs to two projects. This modification lets to specify several projects as a comma separated list in the tag project for one machine, letting it being included in different projects which share the same amazon account.

I've checked that the gem behaviour doesn't change from the default, so if there isn't project specified it still uses all the machines available and if there's any machine without Project specied but it is configured in ec2.yml it skips that machine.