Fix SQLAlchemy support with ORM

[tiangolo]

Fix SQLAlchemy support, including ORM. And update docs accordingly.

• Update jsonable_encoder to support SQLAlchemy models by default, with a new sqlalchemy_safe parameter that checks for keys specific to SQLAlchemy.
• Update tutorial for SQLAlchemy, update code and documentation.
• Use a simple SQLite database for the tutorial, to make it very fast to replicate, while allowing to change a URL to switch to PostgreSQL (or others).
• Add tests for SQLAlchemy tutorial code.
• The new code example can be run as is (copy-paste-run), with a local SQLite database.

This is related to #4 (and it should fix it), might be related to #27 (as the current tutorial is broken). This new tutorial, fix and code should simplify starting with SQLAlchemy.

At some point, it could make sense to add (or replace) instructions for Starlette's integrated database utils, or the new https://github.com/encode/databases and PR #20.

Let's see how it all evolves. For now, this should work in a simple way, supporting SQLAlchemy ORM (that is not possible otherwise). This should also make it easier for people coming from mainstream frameworks like Flask and Django.

[codecov]

Codecov Report

Merging #30 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #30   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          95     97    +2     
  Lines        2502   2552   +50     
=====================================
+ Hits         2502   2552   +50

Impacted Files	Coverage Δ
docs/src/sql_databases/tutorial001.py	100% <100%> (ø)
fastapi/encoders.py	100% <100%> (ø)	⬆️
...st_tutorial/test_sql_databases/test_tutorial001.py	100% <100%> (ø)

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