T 30 schema fixtures

[niden]

Hello!

• Type: new feature
• Link to issue: [NFR]: Schema Fixtures #30

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Schema fixtures. Extend Phalcon\Talon\Database\Schema\AbstractSchema and run talon schema. Configured with schema_source, schema_namespace, schema_output, schema_pre and schema_post.
talon schema writes a directory per dialect: _preSchema.sql, one .sql per table, manifest.json, _postSchema.sql.
Connection::loadSchema() accepts a dialect directory as well as a flat .sql file.
Connection added addTable() and tableExists(); DatabaseTrait added addTable().
SchemaWriter throws SchemaTableDuplicate when two definitions declare the same table.
resources/schema/{mysql,pgsql,sqlite}.sql are now generated directories, and dump_file in the four resources/phpunit.*.xml configs points at them.
bin/talon now honors the Composer autoload path.
docs/index.md was missing MariaDB throughout, six exceptions, and the talon command line runner.

Thanks