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.
I'm not sure what so unexpected about this result. I see only 2 ways to format this. The way given or:
`1.2 ±   1.7   mV`

But the the following should be:
```
>>> print pg.siFormat(1.234e-3, suffix='V', error=2.1e-3, groupedError=True)
 1   ±   2     mV
```
???