Optionally, keep only one video of the same playlist

If the user adds a video which is part of a playlist, all other videos
from that playlist will be deleted from the storage. Users have to
create the entry "extensions.resumelater.oneVideoPerPlaylist" as a
boolean value and set it to "true" to activate the "one video per
playlist" function.
Convert main.js to unix file format
v0.3.7

New version of Add-on SDK
Added videos are always on top of the list.

Even if the video was already in the list.

I think this is much better than putting the new videos to the bottom of
the list (or just keeping them on the same position as they were before
if the video was already in the list).

I think the current videos are much more important than the old ones
that the user want to resume eventually in the future. Therefore I think
it is way better if we put the videos that are important to the user to
the top of the list.

Tell me what you think about this :)
Add licensing information
Fix #17: Cannot Delete Bookmarks

Non-German locale files for the panel were not correctly loaded.
Merge pull request #18 from ico43/new-on-top

Added videos are always on top of the list.
Adapt to per-window private browsing
v0.3.8
Merge branch 'release' into devel

Conflicts:
	README.md
	lib/main.js
Use new require() syntax
Refactor video management and fix #19
Minor refactoring of panel.js
Make height of the video list adjustable by the user
Replace prefService with simplePrefs for oneVideoPerPlaylist preference
Merge branch 'devel' into release
v0.4