Introducing Smack's own XmlPullParser interface which tries to stay as
compatible as possible to XPP3. The interface is used to either wrap
StAX's XMLStreamReader if Smack is used on Java SE, and XPP3's
XmlPullParser if Smack is used on on Android.
Fixes SMACK-591.
Also introduce JUnit 5 and non-strict javadoc projects.
- HeadersExtension.getHeaders() now returns a List instead of a
Collection
- Use XmlStringBuilder in Header and HeadersExtension toXML()
- Add HeadersProviderTest
- Use Smack formatting
Also remove duplicate parsing code regarding SHIM from HOXT
implementation.