useorocos: always export ${PROJECT_SOURCE_DIR}/include/orocos in catkin workspaces if it exists

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
Merge pull request #11 from meyerj/export-source-include-orocos-directory

This enhances the use case that enables building depending packages in the same catkin workspace without relying on catkin to export and import include directories for non-generated headers. It also means you should put your headers into package/include/orocos/package to have a similar layout between the install-space and the source-space.  As a follow-up we would want to update the orocreate-pkg templates to illustrate this layout.