Use JSR 305 Nullable/Nonnull annotations?

Such annotations can help immensely when programming with the API, as it tells the API user what to expect.

Two examples from `DbxClient`:
- `.startGetFile()` can have null as its second argument; it will also never return null; therefore:

``` java
@Nonnull
DbxClient.Downloader startGetFile(String path, @Nullable String rev)
```
- `.getMetadata()` can return null:

``` java
@Nullable
DbxEntry getMetadata(String path)
```