Add an example and stub function for pool

[p-vinayak]

Provides a stub function which returns and error if the user is trying to invoke pool without requiring it.

Also added an example to demonstrate how pool works.

[harsh183]

I really like this. I think this is a good resolution to the pool errors that we ran into earlier and all the build errors are resolved as well. I think this is good to merge.

[dilumn]

@p-vinayak do you know why this pull request include examples/configurations.rb & examples/supervisors_and_registry.rb files?
And why github shows them as empty files?

Have you updated your fork & p-vinayak-pool-example-and-stub-function branch already with Celluloid master?

[p-vinayak]

I added executable permissions for those scripts on my machine because the tests gave build errors since it was unable to run them. I got that same error locally and on travis CI and the tests seemed to pass the tests after I added the executable permissions to those scripts.

I haven't updated the celluloid master branch on my fork yet.

[dilumn]

Change here looks good to me. It is good that we added an example for future users to reference