flow/network: replace NetworkX MultiDiGraph with simple implementation
examples/dataflow: adapt to new simple MultiDiGraph implementation
doc: update for NetowrkX refactoring
test_cordic: stop spewing out numbers
cordic: round() constants if not power of two bitwidth, cleanup, simplify some logic
sim/icarus: add vpi directory to module search path

This allows running the iverilog simulations from the migen top directory
without having to install the .vpi anywhere.
add travis-ci description
README.rst: add restructured text markup
README.rst: add travis-ci badge