Add direct navigation to entity properties

Instead of calling 

```java
Optional<String> name = client.me().select("displayName").get().getDisplayName()
``` 
we should be able to call 
```java
Optional<String> name = client.me().displayName().get();
```
Mentioned first in #57. The specification for this is at https://docs.oasis-open.org/odata/odata/v4.01/os/part1-protocol/odata-v4.01-os-part1-protocol.html#_Toc31358939.
