Build windows and ubuntu binaries

Pull request including:
* an API C file containing one function: get the music key of a set of samples
* build script (in.pro) for Windows VS2010 compiler (under QtCreator), creates a .lib and a .dll files
* build script (in .pro) instructions for Ubuntu linux (GCC), creates .so files
* Ubuntu binary packaging script, testing packages are for the moment located here https://launchpad.net/~julien-rosener/+archive/keyfinder-test
* some integer/float casting which are necessary to avoid VS2010 errors
* a set of external binary libs for Windows compilation (I prefer to put it here because it is very annoying to install libs on Windows)
* build instructions for Windows and Ubuntu in README

Things I think needs more review/discussion than others:
* The API C file only contain one function, we should extend it.
* The kfinder_get_key() function is a copy paste of the first way in documentation, maybe the second way is better.
* All lib dependencies for Windows build are stored in the Git repo, I think it is a good idea, but because of the size we should discuss about that.

Comments:
* I am OK to be the package maintainer of the Ubuntu build, in that case debs will be stored here (if you don't want to create your own repo): https://launchpad.net/~julien-rosener/+archive/keyfinder
* I am also OK to be the Windows VS2010 maintainer but I don't know where to store dlls and libs.