Migration to of-watchdog

An adaptation of of-watchdog.
As per openfaas in future all of the templates going to use the of-watchdog

This PR migrates from go template based on legacy watchdog to go-http-template based on of-watchdog. Along with that comes the below changes

• Faasflow library now completely stateless
• Faasflow puts unique request id in response, as X-Faas-Flow-Reqid Fixes #56
• Flow execution is much faster
• Fix issue with dynamic branch execution Fixes: #62
• Fine tuned API, much easier to chain operations
• Updated README