Updated install

I updated the installation procedure in order to avoid having references to the build tree during normal use of the library (the controllers of course are not included in this).

@traversaro if you want to check the CMake file. There is one point I don't like: I generate first the `.m` script in the build tree and then I install it. Is it possible to do it in one operation (avoiding leaving trash files in the build tree)?

@jeljaik I generated also a `.slx` version for R2014b. I tried to make it back compatible to R2012b but in that way simulink was still complaining. I think that the `.mdl` can work for < 2014b and `.slx` for >= 2014b.
As a last point: how do you check the compatibility version? Because I only see the version in which it has been saved. This question because the `.mdl` says it has been generated from 2014a.

Of course @jeljaik you are the one responsible to merge this request :)

Oh.. I forgot: the share folder is `WBI-Toolbox` with the dash in it (as the project). If you prefer it without dash you can simply change the variable in the CMake file and everything (install and matlab script) should be fine