Windows support

[suchipi]

It'd be nice to support windows, too. I think the only incompatible code is in Job#run:

• These should use the path module instead of template literals:

quinci/src/job.js

Line 110 in 42e031e

const jobDir = `jobs/${taskName}/${commitSha}`;

• We shouldn't use sh to spawn the process on windows:

quinci/src/job.js

Line 116 in 42e031e

"sh",

I'm not sure if windows can run eg. ./quinci/master, it might have to run ./quinci/master.bat or etc.

Also we'd be assuming that git is in the PATH on windows, just like we do for unix.