Use relative paths

This will enable installation of the skin in other directories then the
standard .../skins. However it still expects some layout assumptions to be
true. The expected layout is

+- skins
|  +- chameleon
+- extensions
|  +- Bootstrap
+- vendor
   +- twitter
      +- bootstrap

This patch enables bundling the skins and its dependencies as a package (tar
file or whatever).

Change-Id: I793c9c965e04e84642b7c816ce4c6f3af0177ae9