Allow adding class names to method names for Halstead metric (fix #175)

[devdanzin]

This PR add a --class_names option to radon hal -f that prefixes method names with their class names.

Before:

> python -m radon hal -f .\radon\visitors.py
[...]
    visit_ClassDef:
        h1: 1
        h2: 2
        N1: 2
        N2: 4
        vocabulary: 3
        length: 6
        calculated_length: 2.0
        volume: 9.509775004326938
        difficulty: 1.0
        effort: 9.509775004326938
        time: 0.5283208335737188
        bugs: 0.003169925001442313

After:

> python -m radon hal -f --class-names .\radon\visitors.py
[...]
    HalsteadVisitor.visit_ClassDef:
        h1: 1
        h2: 2
        N1: 2
        N2: 4
        vocabulary: 3
        length: 6
        calculated_length: 2.0
        volume: 9.509775004326938
        difficulty: 1.0
        effort: 9.509775004326938
        time: 0.5283208335737188
        bugs: 0.003169925001442313

I mistakenly committed to master, then created this branch from it. If that causes issues I can send another PR without commits to master.

Fixes #175.