Change default massivity of castle ground tiles.

This was driving me mad.
Reorganize authors.txt file.

It now gives a much better overview of all the copyright holders
plus conact information (the latter is stripped out for the credits
screen in the game). Specialthanks people who have helped the game,
but not contributed code or assets, hence do not hold copyright,
have been extracted into a separate file (which is also read for
the credits screen).
Revert commit 93a94a1ca54bb2f762719f262401ca6786fd4534 and add missing copyright notices.

That commit incorrectly assigned the entire copyright to “The
SMC Contributors” (later changed to TSC Contributors). As we have
officially forked the game, we can’t simply swallow FluXy’s
copyright notice. This commit re-adds it for legal clarification,
and adds the TSC copyright notice below it. Copyright notices of
other external copyright holders have not been touched.

This commit also adds copyright notice headers to all files that
were missing one.
Change fireplant to fireberry in mruby scripting interface.
Clarify asset licensing.
Replace cave_1 music with anhotr by aakburns.
Replace ghost_2.ogg with version of higher quality, “maximie” by Aakburns.
Add “holdon” by Aakburns.
Add “rethink_life” music by Aakburns
Add “murder” music by Aakburns
Add “Arik1” music by Aakburns
Add saving of block position to lvl_1. /cc @datahead8888
Start allowing consecutive level save and load events (unfinished).
Add functions for protecting an mruby object from being GC’ed.
Fix typos
Fix minor issues with consecutive save events
Merge branch 'consecutive-save-and-load-events' into release-2.0.0
Fix incorrect type being passed to Get_from_Position()
Fix bonus box state not being saved.

This doesn’t fix the underlying problem; for some reason, the bonusboxes
are set off 1px when activated, making the save mechanism fail.
Add a climbing box to lvl_5