Humanize member list output

Sometimes when looking at the `members` output when there are a number of nodes running, my OCD side wont quit telling me to fix the alignment. I started writing a patch for formatting output into more readable columns, which I later realized is likely a common want for CLI stuff.

I ended up writing a [little library](https://github.com/ryanuber/columnize) to do it, and this patch uses it for the `members` command. That said, I fully realize that there might be concern linking my library from a Hashicorp project, since you may not want to give some stranger the ability to break your builds, what with the lack of version control in `go get`, so by all means feel free to not merge this in.

I also considered sending this to [mitchellh/cli](https://github.com/mitchellh/cli) as a new output function, but didn't because I don't know that a CLI is the only case where someone might want to use this. I'm open to any suggestions on how to localize control of this.

Anyways, here is a quick example, before:

```
somenode001     10.77.5.101:7946    alive    role=web    Protocol Version: 3    Available Protocol Range: [1, 3]
someothernode023     127.0.0.1:7947    alive    role=db    Protocol Version: 3    Available Protocol Range: [1, 3]
short     127.0.0.1:7948    alive    role=db,datacenter=east    Protocol Version: 3    Available Protocol Range: [1, 3]
abcdef     127.0.0.1:7949    alive    role=queue,datacenter=west    Protocol Version: 3    Available Protocol Range: [1, 3]
xyz123     127.0.0.1:7950    alive    role=cache,version=1,release=123    Protocol Version: 3    Available Protocol Range: [1, 3]
node002     127.0.0.1:7951    alive    role=cache    Protocol Version: 3    Available Protocol Range: [1, 3]
udk-sked-991     127.0.0.1:7952    alive    role=web    Protocol Version: 3    Available Protocol Range: [1, 3]
yetanother00     127.0.0.1:7953    alive    role=lb    Protocol Version: 3    Available Protocol Range: [1, 3]
```

And after:

```
somenode001       10.77.5.101:7946  alive  role=web                          Protocol Version: 3  Available Protocol Range: [1, 3]
someothernode023  127.0.0.1:7947    alive  role=db                           Protocol Version: 3  Available Protocol Range: [1, 3]
short             127.0.0.1:7948    alive  role=db,datacenter=east           Protocol Version: 3  Available Protocol Range: [1, 3]
abcdef            127.0.0.1:7949    alive  role=queue,datacenter=west        Protocol Version: 3  Available Protocol Range: [1, 3]
xyz123            127.0.0.1:7950    alive  role=cache,version=1,release=123  Protocol Version: 3  Available Protocol Range: [1, 3]
node002           127.0.0.1:7951    alive  role=cache                        Protocol Version: 3  Available Protocol Range: [1, 3]
udk-sked-991      127.0.0.1:7952    alive  role=web                          Protocol Version: 3  Available Protocol Range: [1, 3]
yetanother00      127.0.0.1:7953    alive  role=lb                           Protocol Version: 3  Available Protocol Range: [1, 3]
```

There aren't tests because the current tests already do not mandate strict spacing rules for `members`, and that seems reasonable to me. Any `grep`, `awk`, etc commmands should still work as well, there is just added padding in the columns to make them more aligned.