Nbody example improved

The Nbody example has been improved as follows:
- command line options for both particle number and step width
- removed unused variables
- data is aligned using float4 (also I do not see a big performance improvement)
this kernel could be replaced with the [`fill()`](http://kylelutz.github.io/compute/boost/compute/fill.html) algorithm.

something like:

```
boost::compute::fill_n(
    compute::make_buffer_iterator<float4_>(m_velocity, 0),
    m_particles,
    float4_(0, 0, 0, 0),
    m_queue
);
```