Fixed #299. Added HelloWorldPlugin for testing purpose.

This is almost like DelayedPlugin except it does not wait.
We can turn this into a better testing plugin (the configuration
takes parameters, like configuring how many issues to return,
what strings to return, etc). But for now, this is enough for
testing.
Merge pull request #303 from yeukhon/issue#299

Fixed #299. Added HelloWorldPlugin for testing purpose.