Added support for the `-format` argument

- Output formats include json, xml, and text (default)
- Format option added to `members` command
- Reusable output interface for more commands to use as needed
Updated from upstream
Adjusted members.go for new tagging feature from @armon. This makes things a
little trickier mainly because straight key:val pairs should look different in
json and XML, and Go's XML parser doesn't seem to support passing it maps.
Added a TagContainer{} to help XML in creating a parent element for member tags
Removed XML support for output - introduces unnecessary complexity at this point.
Removed xml tags from test structs in output_test.go
Removed remnant test code
Merge pull request #113 from ryanuber/output_format

command/members: Output format