Implemented git versioning

So this implements #40. It automatically versions the shared libraries from any annotated `git` tags:

```
git tag -a v1.0.0
```

It expects semver version tags such as `v1.0.0`. It would be trivial to support `1.0.0` but looking around it seems that most C/C++ projects follow `vX.X.X` rather that `X.X.X` like a lot of `Node.js` stuff.

This determines that the if the project has had a certain amount of commits since the last tag and also if the project is _dirty_ (has modified files), but does __nothing__ with that information. In the future a more robust release could be implemented in the script.

This is pretty brittle and has little in the way of configuration. Ideally we should use `find_program` to work out where `git` is so that users can configure it. This implementation assumes that `git` will be available in `PATH`

Outputs the following on the command line:

```
-- git Version: v[MAJOR].[MINOR].[PATCH]-[COMMITS_SINCE_TAG]-[SHA1][-dirty]
-- Version: [MAJOR].[MINOR].[PATCH]
```

For example, dirty and ahead of a tag by one commit:

```
-- git Version: v1.4.0-1-gf492b08-dirty
-- Version: 1.4.0
```

On a tag and not dirty (this is what a release should be built from)

```
-- git Version: v0.0.1
-- Version: 0.0.1
```