Fix: py3 unfriendly imports in qplot

**Issue**
Python 3 tests fails at

`from ggplot import xlab as xlabel`

in qplot.py

**Problem**
xlab is imported into ggplot as a `*`. This is not
an explicit import and python 3 does not like it.

**Solution**
Change the import statements to import xlab and ylab
from where they are explicitly defined
Merge pull request #234 from has2k1/fix-qplot-import-py3

Fix: py3 unfriendly imports in qplot