added store.fs.rmdir in storage.py to eliminate slow rmdir operation

[hayesgb]

This is a fix to zarr/storage.py that add the fs attribute to store.

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)

[Carreau]

I pushed a commit that should fix the first test failures; I believe that should also fix the subsequent failures as well.

[codecov[bot]]

Codecov Report

Merging #688 (032873d) into master (acf6828) will decrease coverage by 0.00%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
- Coverage   99.94%   99.93%   -0.01%     
==========================================
  Files          31       31              
  Lines       10606    10608       +2     
==========================================
+ Hits        10600    10601       +1     
- Misses          6        7       +1     

Impacted Files	Coverage Δ
zarr/storage.py	99.91% <50.00%> (-0.09%)	:arrow_down:

[joshmoore]

@hayesgb : do you mind adding a test that makes use of self.store.fs?

[hayesgb]

Sorry for the delay in responding. No, I don't mind, but it may be a bit until I can get to it. I added this to eliminate the fallback to a slow implementation of the delete