Percentiles, max outstanding response, 'all mail'

Changed a number of things and I think the script is much better now.
- BUG: Sort was not working; Array.sort() sorts as strings by default.
- Now printing 50th (median) 95th and 99th percentiles, which I think is more useful.
- Cap 'outstanding responses' at some time period (I chose 6 months) since after some point it's probably not going to happen.
- Now searching 'all mail' for getThreads(). To do this we use the GmailApp.search() function. This gives us credit for responses that otherwise we wouldn't see (because you respond and then archive). 
- Structured code so it's more obvious what people have to change.