Added the ability to select individual sources for display

I added the possibility to select individual sources for display.

The list of sources appears below the tags list in the navigation panel.  It is also possible to hide the list of sources (or tags) by clicking at the title 'Sources' (or 'Tags').

###### Possible further improvements:

* I guess, it would be also nice to add some symbol (+ or -) to the Tags/Sources h2 element to show, that the list it hidden/expanded.
* I really think, that 'padding: 5px' should be removed from '#nav-tags li' style, but I am not changing this in the pull request -- let Tobias decide this.
* The mobile setup is not really tested in any way.
* There is a bug -- if the Tag name is long, the unread count for the Tag will be squashed (fixed for sources unread counts only)