Introduce new data models

Based on the following data model, this PR contains the corresponding Java model classes (except the hierarchies downwards LayerDataSource and BaseLayerTheme). This is just a first, rudimentary step in the modeling-process, but i think it is important that we have something to start with.

The model separates data (LayerDataSource) and layout (BaseLayerTheme) and the Hibernate-table-generation (hbm2ddl=create) works fine!

Feel free to discuss the model!

I would really appreciate a merge of this PR!

![image](https://cloud.githubusercontent.com/assets/3939332/3498196/2418d6f4-05f3-11e4-8a88-a55e31b1a3ec.png)

What about a constructor 
```
Layer(LayerDataSource dataSource, BaseLayerTheme defaultTheme)
```
since dataSource and defaultTheme must not be null?