Retry when ADC fails to be read, even on first read
Only read ADC once now that SysFS is working as expected
Add parameter to specificy ADC read repeats
Improve GPIO and ADC peformance, rework imports

- Use lower level file operations
- ADC now defaults to reading from IIO sysfs, which provides the full
  12-bit resolution
- Add scaled mV and volts methods to ADC
- Add properties to GPIO and ADC
- Fix indentation to be 4 spaces
- Import classes via __init__.py
Rework ADC example and add performance test
Add packaging and some initial tests
Merged in upstream changes