Support for loading XML conf via InputStream

We are using this library for Android development. Android makes it very difficult to reference files by path. Instead, Android provides `*Manager` classes, such as `AssetManager`, that will load files for you and return an `InputStream`. 

Example usage: 

```Java
# given Android asset android-project/src/main/assets/ff4j.xml
...
try {
      InputStream ff4jStream = context.getAssets().open("ff4j.xml");
      FF4j ff4j = new FF4j(ff4jStream);
    } catch (IOException e) {
      ...
    }
...
```