removed wildcard handling to fix #71

Following the discussion from #71 here are the changes:
- disabled wildcard handling in Obis identifier comparison.
- added stricter handling on parsing codes to throw an exception if hex codes are used or invalid numbers.
- default Obis constructor now initializes with 0xff to be more in line with string parsing. 
- some more unit tests for obis
- added exception handling for MeterD0 around Obis string parsing. Invalid values will just be ignored but parsing continued. Later we could remove the strong restriction to only allow obis_codes starting with 1 or 2 (especially since all EDIS meter will report 255....) but somebody with a Landis&Gyr E350 needs to test this first.

I tested with my Hager EHZ361 and had no issues and no need to adjust my config file.
Please do intensive testing and let me know any issues.
Change `==` to ` == `?