Make Device properties battery and batterylow optional

• Make Device properties battery and batterylow optional to distinguish between missing values and zero, see issue #11

Fix compatibility with Comet DECT

Thanks to @philippn for pull request #10

Added support for FRITZ!OS 7

• Added support for FRITZ!OS 7 (#9, thanks to @kseb)
• Add support for voltage
• Fix errors and sonar findings

Bugfixes

Fixed tickets

• #7
• #8

Download the new version

v0.3.1

Initial release