add module to let you group objects arbitrarily
add example of custom grouping
fix oversight in docstring
make changes PEP-8 compliant
add a blank line to pycallgraph/grouper.py
add main to __all__ so that flake8 won't complain about unused import

(it gets imported by the examples/graphviz/grouper.py example later)
remove unused import (was copy and pasted from filter example)
Merge pull request #124 from sMAshdot/develop

Specify Custom Groups