Should support database engines other then postgresql

[OriHoch]

Overview

For now only PostgreSQL is tests (and thereof supported).

Plan

• [ ] implement support for other database engines like MySQL or SQLite

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From @OriHoch

reproduction steps

• be a new developer using jsontableschema-sql-py
• look at README, see the exapmle which uses sqlite
• be happy that I can use sqlite and possibly other engines because it uses sqlalchemy
• start using it and developing based on it
• use an 'object' or 'array' type field in a schema

expected

• should work

actual

• error
• looking further, the code relies specifically on postgresql for object / array type field

notes

• I think it can be solved by dumping the values as json into string field
• until it's fixed - state clearly in README and in the examples that postgresql is the only fully supported engine
• I guess, the main issue is not supporting sqlite - which is great to allow new developers to start working quickly. If at least sqlite was supported as well (for development / testing only) - it will be great.

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From @vshih

import sqlalchemy as sa
engine = sa.create_engine('mysql://...')
from jsontableschema import Table
t = Table(SOURCE, SCHEMA)
t.save('test_package', backend='sql', engine=engine)
# _mysql_exceptions.ProgrammingError: (1146, "Table 'mydatabase.users' doesn't exist")

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related #64

[OriHoch]

https://github.com/frictionlessdata/jsontableschema-sql-py/blob/master/jsontableschema_sql/mappers.py#L11