changed messenger to allow multiple start() stop().

Also added more tests on the executor.
Merge pull request #18 from vladimirvivien/pure

removed binary example_executor, added to .gitignore.
Remove unsafe and atomic stuffs. Replaced by mutex lock.
Merge branch 'pure' of github.com:mesos/mesos-go into pure
Updated godep

Replaced mesosphere/testify with stretchr/testify.
Modified debugging messages.
Added Executor.Disconnected().
Added stopped field in executor driver.
Removed locks in sendStatusUpdate and sendFrameworkMessage to avoid deadlock.

Working on a way to serialize events.
added tests for http_transporter, split upid tests.