How would release channels be implemented?

This follows on from #77. It is effectively the approach "(2)".

There is interest from the community in having build channels for:
- RCs
- dev builds (somewhere between a nightly and an RC)
- nightlies

dist.ipfs.io could show nightlies/dev builds as an option under each distribution (we'd have to choose whether to do a grid too, and whatnot), or a "channel" (ie chose from `{release, RCs, dev, nightly}`). It will take some design work and dev work to come up with ideas on how this should look, and then implement it.

We will need to decide:
- How should channels be represented?
- as another dir in the hierarchy? (`///`)
- or as a symlink (it's really a branch, right...)? (`/ -> /`)
- or as link values in the `dist.json` (`{ ... "channels": { "": "", "release": "0.4.2", "RCs": "0.4.3-rc3", ...} }`)
- or as semver suffixes? (`-.`, `0.4.3-rc.3`)
- How should channels show up on the index page?
- maybe "tabs" or "pills" on top of the grid? that we select to change the channel?
- different grids all shown? (this seems very screen real-estate expensive)
- We should display a warning when users download `dev` or `nightly`.
- Basically, we should have a yellow warning for "RCs" and a red warning for "dev" and "nightly", so that people only download them when they're sure.