Implement json writer

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Overview

For now tabulator support only stream.save(format='csv') to csv format. It's pretty easy to implement json writer just porting writers.csv.CSVWriter to writers.json.JSONWriter.

What we're aiming for:

from tabulator import Stream

with Stream('data.xls', headers=1) as stream:
  stream.save('path.json')

And of course it will be a pretty cool and useful feature :+1:

Plan

• [ ] port writers.csv.CSVWriter to writers.json.JSONWrter
• [ ] register new writer in config.py (see WRITERS)
• [ ] add writing tests to tests.formats.json
• [ ] mention writing ability in readme json format section

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For https://github.com/frictionlessdata/tableschema-py/issues/213

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@iamjenmackle Here I've added a simple spec based on the SQL writer spec - #164

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Closed in favor of #199