Fix scrunched header in Safari 6.0.5

Hey @freqdec, I work with @quavmo and we discovered the issue seen in #22 a month or so ago.

I just tried out the fiddle from that issue on my laptop and found it wasn't showing the problem. Until I upgraded to 6.0.5 (I was on 6.0.2). Which might explain any difficulties in reproducing the bug. 

As far as fixing it, I played around with git-bisect and found it was caused in 7e0ad2960e44d0333f6f32eaf38b9a171cf51963, with the "lifted corners" effect. I think the bare minimum you need to produce the problem is
```
.date-picker table:before {
  content: "";
}
```
...which causes Safari to mysteriously give the table header zero width and height.

My solution was to move the :before and :after elements up to the containing .date-picker div, which hopefully doesn't cause any other problems. I moved the shadow a couple of pixels so it maintains the same appearance. 

This has the added effect of removing a couple of pixels from above and below the table, which restores the internal spacing to its appearance prior to 7e0ad2960e44d0333f6f32eaf38b9a171cf51963.

## Before
![screen shot 2013-09-29 at 9 08 58 pm](https://f.cloud.github.com/assets/1041691/1234764/e6919e74-2989-11e3-9c87-456872017a72.png)

## After
![screen shot 2013-09-29 at 9 08 52 pm](https://f.cloud.github.com/assets/1041691/1234765/ef1d53d0-2989-11e3-9e4f-f5445da69c22.png)


This should fix #22