Add Golang vet checker
Install Go 1.2 on the VM
Rename flycheck::hg to flycheck::mercurial

That's just the real name of the tool
Automatically update Go packages on the VM
Fix GOROOT used in Go test cases
Rename go-govet to go-vet

Also rename flycheck-govet-printfuncs accordingly
Do not use go-vet when the vet tool is not available

Check the presence of the "vet" tool with a custom predicate.
Test golint, go vet and go build in a single test case

This test fails. I have no idea why the test fails...
Do not install go vet explicitly

It's part of the binary Go releases, so we do not need to install it
again.
Fix chaining of Go checkers

Do not interrupt the chain if golint or go vet report errors, because
these are not fatal, so the subsequent checkers can still run
Merge go-vet-print-functions test with go test