Add method to split EntitySet

As a developer, I wish I could use Featuretools to split my EntitySet into train and test datasets. Sometimes all of the data will be included when an EntitySet is created, and having the ability to separate the data into independent EntitySets for modeling could be useful.

The relationships defined in the EntitySet could be used to grab all the relevant rows from the related tables.

#### Code Example

```python
train_es, test_es = EntitySet.split(target_dataframe_name="transactions", test_size=0.2, ...)`
```