includes are now installed to global include dest

@eitanme Please Review

Looking at the install dir, we see an install tree that looks something like:
```
pkgA/pkgA/fileA.h
pkgB/pkgB/fileB.h
```
When calling find_package(pkgA), `install/include` is added to our include dirs.  Thus:
```
#include <pkgA/fileA.h>      // Fails
#include <pkgA/pkgA/fileA.h> //Succeeds
```
Looking at other packages, they add their own install rule, such as:
```
install(DIRECTORY include/interactive_markers/
  DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}
  FILES_MATCHING PATTERN "*.h")
```
This manually ensures that things get installed in the right place.

This patch updates the `cs_install` rule so that the `pkg/include` folder installed directly into `install/`.  Yes, this means that bare header files not in a folder will be put directly into install.  This is not ideal, but these files would have been found while in devel space, so the install rule should do it's best to work the same as devel