Implement spelling metric

I have implemented a spelling metric. I will start to use the return
value of the metric compute method to fetch the score instead of using
an instance attribute. The latter approach is a lot more prone to error.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Small fixes on the spelling metric

I fixed the namespace of the Aspell constructor and added to_a methods
to prevent null exceptions if a certain metadata record field is not
available. This saves me from adding an additional null check.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>