httpchecker binary from pandik project

This adds a http checker to the build (it is go - so is built from source and depends on the flapjack structs. 

https://github.com/oguzbilgic/pandik - has the details on how to use (see README.md). 

This had all my changed merged for flapjack support. Agreement is to keep the source upstream in the pandik project for now - only part of the binary build for flapjack. 

This is a CLI tool, libexec seems to make sense.