Fix QString argument substitution

QString replaces only %1,%2 with arguments.
Add warning if bsdtar is absent

If zeal cannot launch bsdtar it loops total download progress bar
endlessly, which looks like it hangs. User must be warned if something
goes wrong, so I added QMessageBox to say about this.

Also, 'program' variable not changed anywhere and can be declared as
constant.
More helpful message about bsdtar missing
Handle GitHub 302 redirects (fixes #102)

 - http://bourbon.io/docset/bourbon.tgz was redirecting to /docset/bourbon.tgz
Show tooltip for external URLs (#82)
Merge pull request #108 from MichaelBeeu/linktooltips

Show tooltip for external URLs (#82)
Find previous with shift+enter (#73)
Moved tooltip code to a little more sensible location.
Merge branch 'master' into findBack
Merge branch 'linktooltips' into findBack
Merge pull request #109 from MichaelBeeu/findBack

Find back