Improved escape and unescape of Android resources with HTML markup

Improved escape and unescape of resources with HTML markup, aligning them to how Android handle them, and adding specific unit tests.
Line too long. Check your [lines are <80 chars](http://docs.translatehouse.org/projects/translate-toolkit/en/latest/developers/styleguide.html#maximum-line-length). You can rewrite this as
```python
target += u''.join([data.forceunicode(etree.tostring(child, encoding='utf-8'))
                    for child in cloned_target.iterchildren()])
```