Windows Makefile support

Added support for Windows/Modelsim.
Icarus won't work under Windows with current version of the GPI code due to undefined VHPI references when linking against Icarus libvpi. However, downgrading to [5d85f65](https://github.com/potentialventures/cocotb/commit/5d85f6530d31706d3a49878d6b878db6408748fb)
allows using Icarus under windows.