Use script tags to demarcate text components

tl;dr: `<script></script>A<script></script>B` instead of `<span>A</span><span>B</span>`.

Old:

![image](https://cloud.githubusercontent.com/assets/6820/3631177/af021294-0eb3-11e4-8b4a-d334cef48dea.png)

New:

![image](https://cloud.githubusercontent.com/assets/6820/3631172/a74a7866-0eb3-11e4-9591-1320ba0c326a.png)

This is nicer in that if you add styles on `span`, your text nodes now won't be affected. Similarly, the target of mouse events will now never be a ReactTextComponent span because the script elements aren't rendered at all.

Test Plan: jest, and some simple browser sanity checking.