Add `json` property to ``Document` class

[bmario]

This property provides a convenient way to access the data stored in a document without the CouchDB internals.

Useful to quickly clone documents.

Closes #51 by superseding it.

[codecov[bot]]

Codecov Report

Merging #52 (fa52571) into master (5a42ab9) will increase coverage by 0.00%. Report is 5 commits behind head on master. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #52   +/-   ##
=======================================
  Coverage   97.15%   97.16%           
=======================================
  Files          13       13           
  Lines         950      953    +3     
=======================================
+ Hits          923      926    +3     
  Misses         27       27           

Files Changed	Coverage Δ
aiocouch/document.py	90.96% <100.00%> (+0.15%)	:arrow_up:

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[adrienverge]

More backward-compatible than #51 :+1: