[FEATURE] improved 3rd party lib finding

This pull request bundles a couple of changes to the process of finding 3rd party libs

 * switch from `CMAKE_FIND_ROOT_PATH` to `CMAKE_PREFIX_PATH`
 * use of real find module for xerces-c and coin-or
 * cleanup of existing find modules

The switch from `CMAKE_FIND_ROOT_PATH` to `CMAKE_PREFIX_PATH` was necessary since 

 * newer versions (`>= 2.8.12`) of the boost find module shipped with cmake had problems with `CMAKE_FIND_ROOT_PATH` on windows
 * we were actually misusing `CMAKE_FIND_ROOT_PATH`. See for instance [this](http://public.kitware.com/pipermail/cmake-developers/2012-January/002850.html) post on the cmake mailing list where the relation between the different `CMAKE_*_PATH`s is explained
means "instead of" ?