Lab fails to start if DKPRO_CORE is not set

```
Lab fails to start if the DKPRO_CORE environment variable is not set. It would 
be good if this could default to some sensible value, e.g. ~/dkpro - probably 
not ~/.dkpro because the lab stores its results there and it would be a pity if 
people could not find them because the folder is hidden.
```

Original issue reported on code.google.com by `richard.eckart` on 17 Jun 2014 at 7:58
