Added {content} tag and fixed several php notices

Hi Mike,

I've had a little snippet of code to add the ```{content}``` tag to the ```[download]``` shortcode for a while now. So the actual download link text can be styled with the editor.

The user will have to wrap it like so: ```[download id="1" format="3"]content <em>goes</em> <strong>here</strong>[/download]```

This is way easier than actually altering the title of the download itself. I know there is a title attribute for the shortcode, but that is not as easy to style.

I've altered the insertion code to actually determine if there is a need for the closing tag and place the cursor in between the open/closing tags.

Other stuff that are fixed/added:
* There were several php notices of uninitialised variables that I believe I've fixed.
* I've decided to remove the View downloads tab when there are no downloads at all.

PS. Sorry, but my editor is set to remove white space at the end of each line upon save... so I've done a sweep on edited files to do that in a single commit.