fix demo and install problems with symlinks in windows

Fixes problems in windows from the use of symlinks into nikola package
 It uses theming.rst to determine if adaptation is needed, if yes then autodetects \ corrects all symlinks in the appropriate directory. 

The goals are
 [x] support running nikola from a clone without installing and without making dirty the WC.

[x] support install from the WC. (at present the WC ends dirty)

As a side effect allows to install from a zip obtained from github