Implemented git versioning

This patch 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]
```
get_git_version CMake function
Merge pull request #42 from mattyclarkson/versioning

Implemented git versioning