Define a helper for linking to Ubuntu man pages

use it like this for http://manpages.ubuntu.com/manpages/quantal/man1/sftp.1.html

```
<%= man_link_to "SFTP man page", "sftp" %>
```

or for http://manpages.ubuntu.com/manpages/quantal/man8/aptitude.8.html

```
<%= man_link_to "Aptitude man page", "aptitude", 8 %>
```