- make u/int parsing more flexible by parsing into the next greater u/int value
  possible when no exact/correct size is given
# e.g. 3-byte u/int will be interpreted as 4-byte, 5-bytes as 8-bytes
- put data parsing into own method _parse()
- add more sml properties:
  . obis - the readable OBIS code
  . valueReal - the real calculated value including scale factor (default item
    assignment)
  . unitName - the readable unit string (e.g. Wh)
- make it possible to assign other sml properties instead of the default
  one (which is the valueReal) by using the "sml_prop" attribute
- update documentation for OBIS property assignment
- fix error log by using logger.error() instead of non-existing self.log_err()
# maybe copy&past left-over
- change _parse() function:
  . put packet size into variables instead of using magic constant
  . changed error handling to try to parse next packet if one failed
  . print out parsing error when parsing failed
Merge branch 'develop' of https://github.com/mknx/smarthome into dev-sml-props-int-fix

Conflicts:
	plugins/sml/__init__.py
- extend tested hardware with power meter from issue #115
- try to read all data available from serial device or network to not have
  left-over bytes available on the next read
Merge pull request #118 from ohinckel/dev-sml-props-int-fix

Sml Plugin: Property assignment, unit names, integer handling, logger fix