main: move console overriding
show stats in options

- longest streak
- time spent in garden
grunt/concat: add new files
Stats: empty() the <dl> before rendering
opts: add activity graph
add jquery.peity.js to grunt concat task
opts/stats: remove longest streak

it's useless
opts/stats: handle empty sessions
opts/feedback: make consistent with other sections
opts/stats: use a line chart for activity
opts/stats: use a bar chart
open session in a new tab if not on memrise
Update CHANGES
change NOTIF_UPDATE to trigger an update notification