Revise and document what exactly is part of the public API

[mortenpi]

• [x] AtomicLevels.Xu probably should not be part of the public API (ref https://github.com/JuliaAtoms/AtomicLevels.jl/pull/46#discussion_r314961417).
• [x] weight in terms.jl is exported, but lacks a docstring
• [x] Exports from levels.jl are completely undocumented

I also took the liberty of creating a 0.1.0 milestone for things we want to get done for the registration release.

[mortenpi]

I found two things that ideally we'd sort out before releasing: weight from terms.jl and everything in levels.jl that is exported, but not documented. @jagot: could you double check that we want them to be exported? Ideally we'd of course have docstrings, but I think it would be fine to release AtomicLevels without having explicit docs for these as well, since they seem pretty straightforward.

The docs for the Xu stuff lives on the internals page, so that is now considered not to be part of the public API.