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.
Fixes bug in relation to airport deletion

If routes exist to or from an airport it should not be possible
to delete this airport. This was still possible, and worse, it
was only possible to delete airports with routes. This is fixed
now.
Fixes bug while deleting rows, fixes gh-15

When deleting a row from the table in certain conditions the program
crashed. This gets fixed with this commit.
Bumped version in README
Release 1.0

This is the first official release, and adds some major functionality
to the program. First of all, you can do the following things now:
- add and remove airports
- add and remove routes
- configure additional parameters for airports and routes
- sort the table by these parameters
Theoretically you can also manage multiple enterprises in this version,
although this feature has not been tested very well and will get more
attention in future updates.
Merge branch 'release-1.0' into development