Revert "Fixed path for common UI-files."

This reverts commit dca25dbba8101903f24e420ceee77b7a6cfcf734.
Revert "Changed served directories after change of path of common stylesheet."

This reverts commit 3e523b0f459d3f3e749024ab34b95c775e46fab5.
Revert "Added serving of files in the hyper/server/ui directory (hard-coded)."

This reverts commit c9e3ac55eb4c00d0e17b05a2c903cb155ff7bc18.
Added CSS placeholder file for the JavaScript Workbench, modify/replace this file to add custom CSS.
Updated comment.
Fixed broken path separator handling on Windows.
Fixes #77.