optional support for posix-spawn

[kenn]

In some large rails apps, fork/exec could be very costly, as you fork a copy of 100MB+ just to exec a simple git command. On a memory constrained environment, posix-spawn is a must.

Take a look at how cocaine optionally supports that.

[mpasternacki]

This is on the roadmap, and will be supported in the next version. It should also significantly improve performance of MiniGit's main client, https://github.com/3ofcoins/vendorificator. Thank you for the Cocaine hint - it's very relevant here.