Merge pull request #616 from ljharb/iojs

Add `io.js` support.

This branch adds support for /

The following features should now work:
 - `nvm install iojs` will install the latest `io.js` version. `nvm ls iojs` and `nvm use iojs` will function as you'd expect.
 - `nvm install node` will install the latest stable `node` version. `nvm ls node` and `nvm use node` will function as you'd expect.
 - In general, a specific `io.js` version can be referenced with the "iojs-" prefix. If `node` were to ever release a `v1.0.0`, `v1.0.0` would refer to `node`, and `iojs-v1.0.0` would refer to `io.js`. In the near future, `node-v1.0.0` will also refer to `node` unambiguously. This applies to all `nvm` commands, including working with aliases and `.nvmrc` files.
 - `io.js`, unlike `node`, does not have a SunOS binary. Please open an issue on if this is actually a problem for anyone, as currently it seems like this won't be for anybody.

**Note**: checksum support upon installation is currently disabled. Relates to
**Note**: installation of `io.js` directly from source (via the `-s` option) is not yet enabled. This will be added soon.

Relates to
Fixes #590