made isUnpackingFromClasspath a configurable option

Signed-off-by: Michael Vorburger <mike@vorburger.ch>
mysql_install_db is now also found if it's only in scripts/ and instead
of in bin/ in baseDir (as that's where it is e.g. in
mariadb-10.0.14-linux-x86_64)
Some (basic) documentation about unpackingFromClasspath, baseDir, and
binariesClassPathLocation.