Add support for fetching deps from multiple solutions

[tylerflint]

When the engine was originally designed, the solutions for vendoring were mostly in disarray. As time has passed, it appears that a few solutions are starting to be standardized:

• GoDep
• Glide
• govendor
• no vendoring solution just go get

The engine should be modified to check for the aforementioned vendoring solutions and run the vendor fetch command accordingly.

[glinton]

Until this becomes automagical, the go engine does allow the fetch command to be specified:

run.config:
 engine: golang
 engine.config:
   fetch: go get -v ./...