Fixed the logic for conditional request.

Using the If-modified-since header option.
Fixed callback for ajax request.

And addressed the state of readyState to avoid accessing the elements
of xhr prematurely.
Fixed logic of the XMLHttpRequest.

Incluing parsing of the response for text and header.
Reorganized the logic of calling Github API.

Addressed multiple cases of responses, including 200 OK response,
304 Not Modified response, both of which are used now in the process
of caching values.
Relocated bg.png into the css directory.

Other pages expect it to be there.
responseText must only be parsed if status is 200.

Otherwise we may be in cases for which the returned text does not
exist or it is incomplete.
Removing console.log debugging statements.

No longer needed now that we are converging to a stable version.
Updated the logic of the groups page.

The function to get the repositories from the cache was moved to the common
ncipnamespace.js file and it is now called from both the index and group html
pages. The logic of the group page was also updated after the model of the
index page, to manage the cases when data has not been modified and when
response from the server is refuse, in which cases the repository information
is obtained from the local storage cache.
Now caching also members and last modified date.

Similar approach to the caching of repositories.

The date of last modification is a separate entry in the cache.