Updates to the Install page

**Non-installable platforms**
![image](https://cloud.githubusercontent.com/assets/25212/3682067/f2c200c0-12c3-11e4-87d5-dc2b22985284.png)

**Installable platforms**
![image](https://cloud.githubusercontent.com/assets/25212/3682069/03c8f43c-12c4-11e4-88e1-6652d7f6aa24.png)

I was looking at the rest of app.js and these routes should probably go under https://github.com/flukeout/appmaker/blob/installpage/app.js#L147 -- at least, it looks like that's the added stuff for running in develop mode.