Add nightly builds

[Daniel15]

Provide an easy way for users to get the latest and greatest version of Yarn!

To be done:

• Set unique version number for dev builds (done: Set unique version number for CI (CircleCI and AppVeyor) builds #1230)
• Create nightly.yarnpkg.com subdomain
• Archive builds from CircleCI (Add webhook for archiving CircleCI build artifacts #1473)
• Archive builds from AppVeyor (done: Add webhook for archive AppVeyor build artifacts #1631)
• Create nightly.yarnpkg.com/latest-version endpoint to return latest version number (analogous to the current https://yarnpkg.com/latest-version).
• Pretty directory listing (maybe tabs per artifact type, and latest release at the top?)
• Create Debian repo for nightly builds, using .debs from CircleCI
• Update end-to-end tests (Add end-to-end tests for Ubuntu 12.04, 14.04 and 16.04 using Docker #1391) to use nightly Debian repo
• Update installation script to have a --nightly flag
• Update docs on site (Document nightly builds on download page website#245)

[Daniel15]

Mostly done, just needs docs. https://nightly.yarnpkg.com/ is up, and there's a Debian/Ubuntu repo at https://nightly.yarnpkg.com/debian/. The install script has a --nightly flag to install the latest nightly build.

[Daniel15]

Done! Documented, and end-to-end tests are using the nightly builds now too.