Replace old 1.5.2 jar link with 1.6 version

The previous permanent link seems to not get updated past 1.5.2, so
hence a new direct link to amazon s3 is needed to get the newest version
of the launcher.
Update server installer for versions past 1.5.2

The download url has changed past version 1.5.2, the current way to
download is by checking a versions.json document for the latest version
and building a download url from that.
Update launcher scripts

* Use relative java path in both launcher scripts
* Remove all custom java options from the client launcher
* Replace -Xms512M with -Xms1024M for the server launcher according to
  upstream documentation
* Remove an unneeded ancient shell compatibility hack ${1+"$@"}
* Update manpages to match new launch commands
Remove username and password options from manpage

With the new laucher script command, unfortunately extra command-line
args have stopped working, so remove those from the manpage (which now
is kind of a pointless doc for the client).
Updated changelog for new intermediate version