drivers/sht1x: Major refactoring

- Use RIOT's GPIO interface to access the sensor to increase portability
- Changed API to allow more than one sensor per board
- Added `sht1x_params.h` that specifies how the sensors is connected - each
  board can overwrite default settings by #defining SHT1X_PARAM_CLK and
  SHT1X_PARAM_DATA
- Changed arithmetic to use integer calculations only instead of floating point
  arithmetic
- Added support for checking the CRC sum
- Allow optional skipping of the CRC check to speed up measuring
- Added support for advanced features like reducing the resolution and skipping
  calibration to speed up measuring
- Allow specifying the supply voltage of sensor which heavily influences the
  temperature result (and use that information to calculate the correct
  temperature)
- Reset sensor on initialization to bring it in a well known state
- Support for the obscure heater feature. (Can be useful to check the
  temperature sensor?)
- Updated old SHT11 shell commands to the new driver interface, thus allowing
  more than one SHT10/11/15 sensor to be used
- Added new shell command to allow full configuration of all attached SHT1x
  sensors
- Removed old command for setting the SHT11 temperature offset, as this feature
  is implemented in the new configuration command