no facility for adding "leafs"

- bug report? no
- feature request? yes
- version: 3.1.0

### Description
Specifically, Class created from reflection can't be added to PhpFile. Generally, you should be able to add leafs beyond mere string creations. 

### Steps To Reproduce
I.E FPhpFile->add(new ClassType), ClassType->add(new Method)

@see SimpleXML
