Humble attempt to add Cygwin support

It is working for me, but GUI never runs, this script is working flawlessly, though and so perhaps worth of merging...
On windows steps are:
1. install Cygwin
2. add adb binaries to system path to be runnable from everywhere
3. download this script, gaia.zip and b2g.tar.gz and run it through Cygwin terminal
It seems like the following commands are the same with "Linux", maybe we can just replace `"Linux")` by `"Linux"|"CYGWIN"*)`.