WIP - Query set reader

[chfw]

With your PR, here is a check list:

• Has Test cases written
• Has all code lines tested
• Has make format been run?
• Please update CHANGELOG.yml(not CHANGELOG.rst)
• Passes all Travis CI builds
• Has fair amount of documentation if your change is complex
• run 'make format' so as to confirm the pyexcel organisation's coding style
• Please add yourself to 'contributors' section of pyexcel-io.yml (if not found, please use CONTRIBUTORS.rst)
• Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #83 into dev will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev      #83      +/-   ##
==========================================
+ Coverage   96.94%   96.96%   +0.02%     
==========================================
  Files          49       50       +1     
  Lines        3203     3226      +23     
==========================================
+ Hits         3105     3128      +23     
  Misses         98       98              

Impacted Files	Coverage Δ
pyexcel_io/constants.py	100.00% <100.00%> (ø)
pyexcel_io/database/__init__.py	100.00% <100.00%> (ø)
pyexcel_io/database/exporters/queryset.py	100.00% <100.00%> (ø)
pyexcel_io/database/querysets.py	95.12% <100.00%> (+0.25%)	⬆️
tests/test_sql_book.py	97.73% <100.00%> (+0.05%)	⬆️

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