Feature init checks

[ckoerber]

Added model, database and migration consistency checks which are run on import of new EspressoDB projects.

This addresses #46

New features

• Added new submodule espressodb.management.checks which gathers cross-checks (e.g., migration checks)
• New project's init function call the run_all_checks() method if the environment flag ESPRESSODB_INIT_CHECKS='1' is set. This function calls migration checks.
• Migration checks compare whether the sate of models, migrations and the database is consistent
• Updated unit tests and documentation for new features.

Changes to existing code

• The previous example folder contained, besides the my_project example project, other unit tests. These tests were moved in the new tests folder.
• EspressoDB's new project init function has been updated to call run_all_checks(). To ensure backward compatibility, projects created before version 1.1.0 will have to manually update this function to make use of the new init check feature (after updating EspressoDB).