add German signals Bü4 and Pf1 (whistle signal)
add German whistle signs to style

z-index is set to 500 in order that a main signal tagged at the same node
has a higher rendering priority
rename Bü1 (DS variant from Deutsche Reichsbahn) to Pf1
rename from Pf1 (DS) to Bü4 (DS)
add German Bü4 signal with two stripes
add German Pf1 signal with two stripes
add German Bü4 and Pf1 with two stripes to style
Merge remote-tracking branch 'upstream/master' into pfeiftafeln-icons
Merge branch 'master' into pfeiftafeln-icons
Merge pull request #71 from Nakaner/pfeiftafeln-icons

Added german whistle posts (Pfeiftafeln)