Use top offset and height of contentSelector to determine pixelsFromWindowBottomToBottom

> This fixes a bug where two instances of infiniteScroll of different
heights won't be triggered at their respective navSelector elements
until you get to the bottom of the window.

Use case:

1. Have a sidebar and content with different subset of posts
2. Each column needs to independently scroll
3. Before patch, the next pages would be reloaded only when I reach the window bottom, even if one of the columns was 500px shorter
4. Now each column will reload independently when it's navSelector is reached

Tests aren't affected and my own use case now works flawlessly.