examples: Rename Go files to avoid build failure on Heroku

Currently the build fails with:

    -----> Running: godep go install -tags heroku ./...
    # github.com/cactus/go-camo/examples
    examples/go-hex.go:11: CAMO_HOST redeclared in this block
            previous declaration at examples/go-base64.go:11
    examples/go-hex.go:13: GenCamoUrl redeclared in this block
            previous declaration at examples/go-base64.go:17
    examples/go-hex.go:26: main redeclared in this block
            previous declaration at examples/go-base64.go:30
    godep: go exit status 2