fix(subheader/sticky): Stick subheader to content instead of parent.

[topherfangio]

The $mdSticky service used to stick it's contents to the parent of the <md-content> container. This resulted in some odd styling issues and the inability to have two lists with sticky subheaders side-by-side.

This fix appends the subheaders directly to the <md-content> to fix this issue.

Also, updated subheaders to use <div> tags instead of <h2>.

fixes #3535

[topherfangio]

@jelbourn @ThomasBurleson @rschmukler Can you guys please review this and let me know if you see any issues with the updates? This is related to the discussion we had in Monday's meeting (#3535), but I found a way to do it without the visual glitch I mentioned.

[jelbourn]

I know it was like this before, but could you change this?
self.next.top - scrollTop <= 0 is a less readable way of saying self.next.top <= scrollTop

[topherfangio]

@jelbourn @ThomasBurleson As requested, I have refactored the onScroll() function and it's comments quite a bit to increase readability/understanding. In particular, I replaced the if/else if blocks with more simple if/return statements to more clearly separate the different cases and remove some duplicate code.

Can you two please take a look and provide feedback? It may be easier if you look at the whole file rather than the diff so you can see the overall structure.