format: gofmt the following files

src/mesos.apache.org/example_executor/main.go
src/mesos.apache.org/example_framework/main.go
src/mesos.apache.org/mesos/executor.go
src/mesos.apache.org/mesos/scheduler.go
Merge pull request #2 from yifan-gu/gofmt

format: gofmt the following files