Definition of basic Git data model

The goal of this task is to define a model of data for Git data source. This basic model will comprise those entities that not require a context to be generated.

The model defined by CVSAnalY to store Git data can be used as a starting point. This model defined some entities such as:

* Commit
* Action
* File
* Repository
* Tag
* People

A diagram of the model can be found [here](https://github.com/MetricsGrimoire/CVSAnalY/blob/master/db/cvsanaly_model.svg).