fix: incorrect DDL generated when using server_default (#209)

[trevor-pope]

Added parenthesis to enclose DEFAULT values of CREATE TABLE statements, and changed the order of NOT NULL and DEFAULT statements to match the specifications of Spanner.

Fixes #209

Could also probably add some tests for server_default.

I also realized that the intended way is to use sqlalchemy.text() to render default values, so the other problem mentioned at the bottom of #209 is not true.

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[IlyaFaer]

@asthamohta, okay, I've fixed everything here.

It appears now linter is checking long_description of the package. In the main python-spanner repository the README file is used as a long description.

Also it requires to use .rst format README file instead of GitHub default .md (otherwise list_setup_py will still fail). So, I've converted the README file as well.

Let's merge the PR, and I will add tests for it separately (not very convenient to work in client's fork).