Update signs.rst

Add warning about non-existent icon files
Fix reference to stochastic command line option
Revert "Use protocol-relative scripts in index.html"

This reverts commit fabc244c916815c17ca3fcd76dcd75084ff43dde.
Using protocol-relative scripts broken users who don't use webservers
(i.e. file:/// )
Merge pull request #937 from tswsl1989/patch-2

Fix reference to stochastic render command line option
Merge branch 'master' of git://github.com/overviewer/Minecraft-Overviewer