Added a couple of more properties found with FritzOS 7.29

[philippn]

Hi there,

I'm using the Java api with a couple of Hkrs, that are part of a group. With this usage pattern, I found the following properties to be missing:

• Element hkr
• Element txbusy
• Attribute synchronized

I hope you find this pull request useful.

Thanks and kind regards, Philipp

[kaklakariada]

Thank you for your pull request! PR #46 already added two of your new attributes. I added the hkr element in https://github.com/kaklakariada/fritzbox-java-api/commit/39368363f93c9f6227f513bb252ea0639b17ef7a