Not be parsed properly with hyphen or underscore are used to the package name

[mkouhei]

Traceback (most recent call last):
(snip)
  File "/path/to/venv/lib/python3.4/site-packages/py_deps-0.4.0-py3.4.egg/py_deps/deps.py", line 249, in draw
    return graph.router(self.traced_chain, draw_type=draw_type)
  File "/path/to/venv/lib/python3.4/site-packages/py_deps-0.4.0-py3.4.egg/py_deps/graph.py", line 16, in router
    linkdraw = Linkdraw(chain_data)
  File "/path/to/venv/lib/python3.4/site-packages/py_deps-0.4.0-py3.4.egg/py_deps/graph.py", line 117, in __init__
    self.descr = "%s dependencies" % chain_data[0].name
IndexError: list index out of range