WIP: Modification for the working of social-help activity.

This PR introduces the changes required in sugar for the functioning of the social-help activity. The changes are self explanatory and I'm adding a few comments (GitHub ones) in the diiff to explain some of the things. 

This PR is a submission to one of the GSoC 2014 sugarlabs projects.

PS: I still have to run pep8 and pyflakes on all files.
Here's what I'm thinking we should do: We'll have a json file on the server and our admins can update it. Also, well not check the presence of an activity category and we'll just show the launch button in every activity.

Now, when the user clicks the button (or presses the keyboard shortcut), we'll check the version of the existing json file in `.sugar` directory and if a newer version is available (which we'll determine by date), we'll first download and save that new version internally. Then, we can find out the category name from the json file and open the social-help activity to the URL. If say, we don't have a category for that activity, we can just show a 404-wiki page that displays information about the unavailable activity and options available to the user (like reporting to the admin). I think this is a pretty good scheme. What do you say @godiard ? I can code it up on this weekend.