Add uncertainty

Add the possibility to print an uncertainty in siFormat and ValueLabel. The uncertainty is printed with one significant number and the precision of the value is adjusted appropriately. Also add unittest for siFormat, siEval, siScale and ValueLabel. In addition correct a little bug found with the unittest, the character µ is not the same as μ but both should be recognized by siEval.
This can cause some unexpected results:
```
>>> print pg.siFormat(1.234e-3, suffix='V', error=1.7e-3, groupedError=True)
 1   ±   2     mV
```