A problem with installing include files

In my recently merged pull request (#586) I added a new file 'common/include/pcl/common/impl/**detail**/accumulators.hpp'. Unfortunately, it turns out that it is not installed in the proper location ('pcl/common/impl/**detail**/accumulators.hpp'). Instead the installation script deploys it as 'pcl/common/impl/accumulators.hpp'.

Apparently, this happens because in ['CMakeLists.txt'](https://github.com/taketwo/pcl/blob/b5de9e3b7ceb12549fa3cc2d553130b19e5eddca/common/CMakeLists.txt#L136) I added it to the `common_incs_impl` list, and all of the files from that list go to 'common/impl', no matter to which subfolder they actually belong.

On one hand, I could get rid of 'detail' folder. On the other hand, a few lines could be added to 'CMakeLists.txt'  to handle this folder properly. The question is what will be the right way to resolve this problem.

The reason I put the file in 'detail' is because a) the code inside is in `detail` namespace b) to signify that it is by no means a part of public API. This sort of namespace/folder policy is quite common in Boost for instance. On the other hand, we do not have anything like this in PCL.

Any thoughs? PCL is broken to some extent now (i.e. in compiles and installs fine, but using this installed code from other project will lead to an error), so this issue is somewhat urgent.