Keyboard shortcut 'a' for 'Add entry' on timesheet.

I was trying out uberzeit and adding some time entries, but I got a bit annoyed that I had to take my hands off the keyboard to reach for the 'Add entry' button every time I try to add a lunch break.

With this keyboard shortcut, you just have to hit 'a' (no modifiers), enter some more times and then hit enter, rinse, repeat :)

I'm not sure if you want to handle keyboard shortcuts centrally at some point in the future, I notice some keyup events being used but I believe (?) they're in areas of the code that isn't used right now (time_sheets). I tried to add this in the most specific bit of code I could find.

I'd also like to add 'n' for 'next day' and 'p' for 'previous day', but I'll make this very very small in case you don't like it :)