Format RAM/disk numbers properly, and comma separate IPs

- space-separating IP addresses makes this very hard to parse, because it means each row has variable size number of columns (servers with 2 IPs have one extra column).  With comma-separated IPs this is a no-brainer to parse in awk or cut.

- numbers such as RAM and disk should be always right aligned for readability, and should support non-integer values for RAM, since the smallest instances offered by Joyent can have < 1GB RAM size.
Adding flavor column to knife joyent server list

- it's based on attributes Hash that seems to contain "package" attribute.
Making backward compatible and tags optional

- ensuring attributes method exists for backward compatibility
- removing redundant .to_s
- only printing tags if SHOW_TAGS env is set 

The reason for optional tags is that currently, servers that have tags defined, result in incredibly long lines, and make the output hard to read. Now that flavor is part of the default listing, I thought turning off tags by default is a good idea. One just need to set SHOW_TAGS=1 before running to show them again.
Merge pull request #65 from kigster/master

GH-65 Format RAM/disk numbers properly, and comma separate IPs