Update README.md
fixed check for cflow, dot on PATH.

check using which, so site-independent and gets path, whatever that might be.
converted indentation from tabs-> spaces, see http://www.python.org/dev/peps/pep-0008/#indentation
removed globals
rewritten most of the code

using subprocess instead of os
replaced with regexp node processing (previous was buggy)
started using networkx as well (optional in future)
improved structure by putting things in more funcs
option for underscore replacement, for svg inclusion in latex
avoid duplicate edges (leads to a huge graph providing no extra info)
more verbose debug mode
dot directly exports to svg, pdf, etc, no reading back in python