Typo: database final code/tutorial mismatch

The tutorial included code to connect to the database that was not the same as in the final `app.py` included in the package. This broke the tutorial immediately, specifically parts that had to `/post` anything.
Added `init_db()` to bottom of `app.py` in tutorial

To ensure fresh database starts every time one runs the server, `init_db()` was added to the bottom of `app.py` in the tutorial. This is also matches what the final result in the package looks like.
Merge pull request #4 from brianclements/master

Final code/tutorial mismatches