Date picker has subtitle active when re-opened

[frobinsonj]

This causes the incorrect section to be highlighted (see images below). This is a result of these two lines which are run on the dialog being hidden. I fixed it myself by swapping round the actions. Not too sure about creating pull requests etc. so I thought it was best to put this here. Thanks :)

md-date-time-picker/src/js/mdDateTimePicker.js

Line 277 in 29f967d

title.classList.remove(active)

md-date-time-picker/src/js/mdDateTimePicker.js

Line 278 in 29f967d

subtitle.classList.add(active)