Modified pin.pp to allow extra pinning types

Ciao Alessandro,

  I've modified pin.pp to receive parameters in a different way. It can now receive "type", "value" and "priority".

  This allows:

 * Use a single template, making it easier to maintain it.
 * Specify another pinning type, "origin".

Also modified the filename generated to be "$title-$pin_type". This allows for better naming in these cases:

* Package * : filename name would be "pin-*" in the old version
* Package one two three : filename name would be "pin-one two three" in the old version
* Two pinnings (i.e. different priorities for releases) would end up overwriting the same file.