WIP: Add custom json encoder and decoder

[constantinpape]

Add custom JsonEncoder / JsonDecoder to properly handle nan and inf attributes, see #412.
Also adds a test to ensure these values are properly encoded / decoded.
This is still WIP because:

• I haven't tested the solution myself yet.
• Potentially one could simplify the code here and here with this solution, but I am not sure this is a good idea, because I don't know if we can always assume that the metadata passes through zarr.util.json_dumps/json_loads
• Potentially one could also add json encoding for numpy dtypes; this is an error that always annoys me a lot, but it's unrelated to Special treatment of NaN in JSON user attributes #412.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[jakirkham]

Should the encoder/decoder bits be moved to Numcodecs?

[constantinpape]

Should the encoder/decoder bits be moved to Numcodecs?

Happy to move it there if you think that would make more sense. Would you put it here?

[constantinpape]

Tests pass, so the solution seems to be viable.

[joshmoore]

Sorry, @constantinpape. Somehow I missed this when reviewing #933 which maybe should been considered together.

[joshmoore]

Zarr/JSON encoding came up again yesterday during the community call. I will try to resolve the conflicts here and see how things stand.

[jhamman]

This has unfortunately gone stale. Will it be revived or should we close this?

[jhamman]

Closing this out as stale. We'll keep the feature request open.