Use relative paths

This will enable installation of the extension in other directories then the
standard .../extensions. However it still expects some layout assumptions to be
true. The expected layout is

+- extensions
|  +- Bootstrap
+- vendor
   +- twitter
      +- bootstrap

This patch enables bundling the extension and its dependencies as a package (tar
file or whatever).

Change-Id: Ib2d68a3ba3e7e27fa2c0da65c82586a00aaef713