Fsspec writing

[martindurant]

[Description of PR]

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)

[codecov-io]

Codecov Report

Merging #630 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #630   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines        10006     10040   +34     
=========================================
+ Hits         10006     10040   +34     

Impacted Files	Coverage Δ
zarr/convenience.py	100.00% <100.00%> (ø)
zarr/core.py	100.00% <100.00%> (ø)
zarr/storage.py	100.00% <100.00%> (ø)
zarr/tests/test_storage.py	100.00% <100.00%> (ø)

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[martindurant]

(more documentation to come)

[martindurant]

(tagged three for review, one would do)

[Carreau]

+1 feel free to merge, I did one suggestion, but otherwise LGTM.

[martindurant]

Thanks, @Carreau