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.
Compare these:
```
>>> print pg.siFormat(1.234e-3, suffix='V', error=1.7e-3, groupedError=True)
 1   ±   2     mV
>>> print pg.siFormat(1.234e-3, suffix='V', error=1.7e-3)
1.23 mV ± 1.7 mV
```
The thing I was not expecting is that using groupedError changes the precision (and the spacing as well, but that's a different issue).