New PR  which Fixes #900 for comment notification to users.

I am closing  the PR #1106 and creating this new one as there will be lot of changes and that was on old develop version. I have rebased with latest code and tried to care of all the comments you had given in PR #1106.
The points covered are as below.
1>Restored the key names back with '-' show-command-promp in settings panel.
2>Changed the entire design and moved the entire email creation and decision logic based on subscription to   the server side in Notebook.comments.R and rcloud.support.R .Now the parameters( username, gistname, notebook title, and notebook owner etc) are fetched at the server side).
3>Reused rcloud.config.get.user.option function and got rid of the new function which I had added.
4>Exposed the get.user function from Carlos's rgithub package and removed the function I had written to get the user email from github .
@gordonwoodhull Please review and let me know if this is how you expected.
5>Made the "From emaild address" and "smtp server details" configurable through rcloud.conf.

The emails look like this
![leaflet1](https://cloud.githubusercontent.com/assets/391355/5274662/206388b4-7a66-11e4-9a2a-c67557a69e8e.png)
![leaflet2](https://cloud.githubusercontent.com/assets/391355/5274663/223bde48-7a66-11e4-857d-2766445693e1.png)







should default to false (opt-in)