Added import and export, changed item storage

Adds: now each master view row shows the number of items done and total
items for each list.
Adds: export data to external storage and import data from external
storage.
Adds: creating a new list now automatically opens it.
Change: items too are now stored with JSON encoding.
Change: icons for list options have been updated (to look more similar
to other android icons) and are now BIGGER.
Change: a little engineering in master activity code concerning loading
and storing data.
Change: detail row is now composed by a textView (item name) and and
imageView (item check).
Change: long click on an item now starts the contextual action bar,
letting the user select groups of items via single click and delete the
selected items via a button in the action bar (on top right of the
screen).
Removed: it's no more possible to modify an item name.
Bugfix: list deletes and modifications now affect the correct list.