use semver

The project starts using semver today. The first version is bound to be
2.0.0 though each component has its own versions (for instance, libleela
is in version 6.3.0).

Since leela has many components we defined the following rules to change
the project version.

* Only one version number changes per release. Priority goes from left
  to right. In other words ``major > minor > patch``.

* Numbers increment by intervals of one;

As an example suppose the project is at ``x.y.z`` and we have the
following changes:

* ``libleela a+1.b.c``

* ``warpdrive d.e+1.f``

The resulting version is going to be ``x+1.y.z`` since major has higher
priority than minor and patch versions.