Release 1.4.16, 1.4.17, next

Added `next` to point to a not-yet-released version.
It's not listed in the dropdown, but navigable via an URL.

The point of having `next` folder is to be able to push things to
usermanual repo before the code lands in AT/AT main repo.

Also, a change to have both `1.x.x` and `latest` folders at the same time
in the repo, so that `1.x.x` is frozen, hence it's impossible to
inadvertently introduce some new development too early in `latest` folder
and making it later wrongly appear in `1.x.x` after next usermanual bump.

Any new development's documentation should be added to the `next` folder,
and then during the release time it'll be copied to `latest` accordingly.
Add .gitattributes
Add placeholders to jade files

So that the release script knows what and where to update
Add release script

The script updates jade files, copies things into proper places, and
creates a git commit.

The workflow from now on should be as follows:

- If the current stable version is `1.4.17`, there's a folder named `1.2.3`
  and also folder named `latest` that both contain the same documentation.

- Developers commit their development to the `next` folder (NOT to
  `latest` -- latest should not be touched manually).

- Because `next` and `latest` are separated, documentation updates to
  not-yet-released version of AT can be safely pushed to the next folder,
  without the need to wait with merging until the framework has been
  released.

- During release time of `1.2.4`, contents of `next` are copied to the new
  folder `1.2.4` as well as to `latest` (forced copy).

Note that if there were no changes in MD files, then there'll be zero diff
on the `latest` folder, just the `1.2.4` folder will be created.

Usage example:

    ./release 1.5.1

In case you've done something wrong (forgot sth etc.) and you want to get rid
of the broken release, just drop the commit:

    git reset --hard HEAD^
Make `next` the default for development
Release 1.5.1