ninja: move out of pythonPackages

ninja is a build system written in C++ that just happens to use python
to build/install *itself*. It is not a "python package".

After this commit, ninja will be at pkgs.ninja instead of
pkgs.pythonPackages.ninja.
ninja: add description and license
ninja: use version attribute

...to avoid the risk of downloading a different version than the package
name suggests (i.e. on the next version bump).
Merge pull request #521 from bjornfor/ninja

Ninja: some small changes