Choose database to load upon initialization

The class 'Database' gets initialized with a String now, pointing
to the file to load. This is needed for testing where a database
with default values will be used.
Deletion of main hub gets prevented

The user must not delete his main hub. This gets enforced with this
commit by introducing an additional check.
Database for testing created

A database has been created and filled with dummy values for testing
purposes.
Removed one entry from test database

This was necessary to allow automated tests with Strings containing
numerical values.
Creation of routes adjusted, fixes gh-4

While creating routes the airports where not loaded from the model
but instead newly created. Because of this the table model was
referencing different objects for the same airport. The fix for
this bug is obviously to use the methods provided by the model to
access airports.
Removed one entry from test database

After the deletion of one airport a route to this airport was not
deleted, which caused some errors in the tests. This has been
corrected.
Tests for database operations, closes gh-12

After several irregularities have been detected in the database
these tests got written to validate the correct behavior of the
application. Minor modifications have been made to the model to
enable testing, and some bugs got fixed along the way.
Merge branch 'tests' into development

A first set of tests has been implemented. It checks mostly database
related operations and is supposed to validate the correct behavior
of the model.