run 'go vet' as part of test suite; colorize test suite

From 'godoc vet':

    Vet examines Go source code and reports suspicious constructs, such as
    Printf calls whose arguments do not align with the format string. Vet
    uses heuristics that do not guarantee all reports are genuine problems,
    but it can find errors not caught by the compilers.
Merge pull request #3 from cloudfoundry/go-vet

run 'go vet' as part of test suite; colorize test suite