Fix iOS Example App Detail Label

The iOS test app currently:
 1. Doesn't top align the username with the profile image
 2. Has inconsistent padding around the detail text label

#### Old Spacing
![old spacing](https://cloud.githubusercontent.com/assets/48065/5194545/6d7aadf6-74c3-11e4-9e5d-deb0a7f375eb.png)

#### Fixed Spacing
![fixed spacing](https://cloud.githubusercontent.com/assets/48065/5194520/1ab203d0-74c3-11e4-93d9-df60c8c1dcb6.png)


Calculate the height for the `detailTextLabel` separately to allow the frame to be set accurately without having to worry about the `fmaxf` in `heightForCellWithPost:`.