Add version to metadata

[jklaise]

Fixes #236. Takes the same approach as SeldonIO/alibi#481.

[jklaise]

It appears that due to the current tests of comparing the meta attribute directly to a literal dictionary, the literal dict would need to include "version": __version__ for every test.

It might be better to factor out these meta dictionaries and use them when initializing the detectors, though this would still necessitate including "version": __version__ for each one if we keep the same tests.

[jklaise]

Latest commit adds __version__ entry to the metadata tests. This seemed to be the easiest way to fix the tests without doing any refactoring.

[ascillitoe]

@jklaise, @arnaudvl: I've merged this, but a minor follow-up thought, do you think we should move the save/load instructions from "Getting started" to a separate "Saving and loading" page like in alibi? For consistency with alibi but also because I wonder if we could do with adding a warning about different alibi-detect versions like in alibi, and this would start to clutter the "Getting started" page?

[jklaise]

Sounds like a good idea.