Add fnenv.Notifier interface + restructuring of fnenv package

[erwinvaneyk]

• Added initial interface for the fnenv.Notifier (which will allow the workflow engine to notify environments implementing the interface of future invocations)
• Re-added the disappeared gofmt and govet checks to the travis config
• Renamed a couple of variables to idiomatic go
• Renamed fnenv/test to fnenv/mock to better indicate the purpose of the package.
• Renamed a couple of files in fnenv to reflect actual contents
• Moved the fnenv interfaces (runtime, resolver) from the api/function to the fnenv package
• Added documentation throughout the fnenv package and subpackages.
• Replaced map with syncmap implementation in Fission Proxy due to potential concurrent read/write issue
• Moved status shortcuts to the status-struct rather than adding them to the flag field. This eliminates the need for the awkward wi.Status.Status.Completed() in favor of wi.Status.Completed()