DOCS: Replace copyright notes with trademark notes

As per issue #47, usage of copyright notes is wrong there.
ALL: Remove wrong-headed copyright note

See issue #47.
Instead, we list the trademarks in the AUTHORS now.
ALL: Update the copyright header

Replace "this software" with "xoreos" to make it more explicit
and link to the GPL on the web instead of suggesting to write
a letter to the FSF.
BUILD: Separate build fields in the version with a .

Per 10. of Semantic Versioning 2.0.0: build metadata.