Fix scrunched header in Safari 6.0.5

For some reason, adding generated css content a :before
to a <table> element caused the header to be squished into
nothingness. Only in Safari.

This commit moves the 'lifted corners' effect from the
.date-picker table to the .date-picker itself, which solves
the Safari problem without seeming to cause any other problems.

This should fix #22
Merge pull request #24 from tjgrathwell/fix-header-in-safari

Fix scrunched header in Safari 6.0.5