Probably the result of a bad merge

* Make the declaration of fct and it in the constructor Collection,
 instead of adding them from the outside
* Add a function to flatten a list containing collection (solves the
 radioob search crash)
* Better display of collections in the "ls" command (and display both id
 and title)
* The "cd" command goes to the root of the path (like the UNIX cd)
* Move the Video object of canalplus in a correct path
* Make Collection iterable
* Add comments to CapCollection
* Cache the result of fct in a Collection; it is only called once
* CollectionNotFound errors can be more explicit by providing a path
* Require utf-8 in collection paths
* Code cleanups

If the user starts with ^ or ends with $, do not add them. This allows
using only $ or ^.
If it's not a string, don't do anything to it (it has to be a regexp, or
mimic one).
Use search() instead of match(). They are the same, except match implies ^.

This does not break any module, it only adds more possibilities.

closes #661

try_drop_tree() was only removing the first result.
closes #725

There is no reason to hide it from anyone, the GPG default is not
appropriate in our case.

Remove it only just before extraction

So that the user can easily check it is legit through another channel.

stderr is shown and an exception is raised if GPG does not exit
successfully.

Useful for stupid users (like me) who delete the directory.

So that deleted/renamed files upstream are actually deleted.
Also check that the proper module directory was created after extraction.