v2.1.2

this is mostly bugfix-release with the following changes:

• more explicit coding
• a lot less compiler warnings on PC, esp, atiny and teensy
• code compiles on PC and should be easily ported to raspberry or other linux-boards
• unittests on PC!
• therefore a lot of bugfixes -> hub.attach, esp8266, bea910, ds18b20
• even more bugfixes for ds18b20-family (degC<=0)
• a little bit simplified interrupt-handling