Feature/counting algorithms

This feature contains the following modifications:
* Re-factor counting algorithms: `num_points`, `num_segments`, `num_geometries` and `num_interior_rings`: this refactoring involves moving common code to a separate file `algorithms\detail\counting.hpp` and a new namespace that I called `bg::detail::counting`. As part of the re-factoring, the `num_points` code in the `multi/algorithms` directory was also moved in the `algorithms` directory.
* Add variant support for `num_interior_rings` and `num_geometries`.
* Add unit tests for `num_interior_rings` and `num_geometries`.
* Implement `num_points` and `num_segments` for boxes of any dimension: to do this I implemented the Boost.MPL-based metafunction `ipower` in the `bg::util` namespace that computes the power of an integral constant at compile time (there is also a unit test for this metafunction). As part of this change, the unit tests for `num_points` and `num_segments` now test for boxes of dimensions 3, 4 and 5.
either we have it on two lines 
static inline
return_type apply

or all on one line
but please have the return type before apply (as long as it fits 80 chars)