Always export ${PROJECT_SOURCE_DIR}/include/orocos

This is a follow-up of #5 and #8.

With this patch, the header files in `${foo_SOURCE_DIR}/include/orocos` will always be made available to other catkin (and rosbuild) packages that `orocos_use_package(foo)` within the same or overlayed catkin workspaces, if this directory exists. This is similar to the layout for installed packages and mimics the design of catkin, where headers are also shared directly from the source folder when working in devel-spaces. Until now, only automatically generated headers in the catkin devel-space have been exported by `orocos_generate_package()`.

Note that these headers still need to be installed manually or by using the `orocos_install_headers()` macro if the package is supposed to be installed.

Of course one could also share include directories using catkin directly by calling `catkin_package(INCLUDE_DIRS ...)` and `find_package(catkin REQUIRED COMPONENTS foo)`, but I don't like this parallelism for pure Orocos packages that just use catkin as a build helper.

Another minor update: I changed the cache type of the `*_EXPORTED_OROCOS_TARGETS` variables to INTERNAL, so that they are always overridden for the case there have been updates in the cmake files or the include/orocos directory has been newly created without a workspace cleanup.