Various changes to routes, fixes gh-1

There was an issue with a SQL statement regarding the update of
routes which gets fixed with this commit. Also some changes have
been made to the way routes get displayed, most notebly the font
has been slightly adjusted, and in the TableModel the conversion
of Strings to Integer has been improved.
Countries added to database

The database contains all countries now. This has been done to
provide more features in the future, for example country flags
where a country is set (especially for airports).
Adjusted creation of enterprises

Due to the addition of all countries to the database no need exists
for the user to enter a country himself, so the dialog has been
changed to provide a list of countries instead of an input dialog.
Filter destination airports when creating routes, closes gh-3

When a new route gets created the list of destination airports gets
filtered, and all airports with an existing route to get removed.
Prepared database for gh-2

The enterprise loses its foreign key relationship with country, and
instead links to an airport where the enterprise has its main hub.
Preselection of main hub when creating routes, closes gh-2

When a new route gets created, the main hub of an airline is now
the suggested selection for the origin. To provide this feature
several changes have been made to the way enterprises are handled,
and to the flow of dialogs. Most notebly an enterprise does not
have an home country anymore, but a main hub from where it operates.
Accordingly the dialog for creating an enterprise has changed, and
of course the route creation dialog itself.
Table updates changes to model automatically

The table model has been adjusted to update its rows automatically
when a value gets modified. E.g. if several routes to an airport
exist and one value of it gets changed, this change is immediatly
displayed in all rows where this airport is listed. Before this the
update took place only when the user selected the accordings rows
manually.
Prevent NumberFormatException in table

If an empty String was submitted (i.e. empty field) the exception
in the title was thrown. To prevent this empty Strings are changed
to "0".
Routes get highlighted based on scheduling, closes gh-5

By introducing a custom TableCellRenderer it is possible to highlight
routes according to their scheduling. Scheduled routes are displayed
with a green background, unscheduled routes with a red background.
Footer removed from routes panel

Until proper filtering has been implemented the footer has been
removed from the routes panel to enable the work on the beta
release.
Create README

The README contains information for the user on how
to use the application.
Release 'Beta'

This is the beta version of the application. Although still in development
and testing it supports the following features:
- add routes
- add additional parameters (sizes, distance, ...)
- sort by
Theoretically this is all that is needed to maintain your routes in the
game, but of course additional features are planned, most notebly several
tweaks to the UI.