Implement dynamic version number

Here is my approach of a simple implementation of what we have discussed in #26. When building from a git repository the version number will be fetched using `git describe`.

![img](https://dl.dropboxusercontent.com/u/743689/PlasmaShopRevPR.jpg)

This requires a tag to work or git describe will return a fatal error. For testing I created a local tag. Currently this only supports annotated tags but support for lightweight tags can be added by calling `git describe --tags`.