Improve social.js UI to work on iOS 7 and on Android (Ti 3.2.0)

This pull request bundles some changes to improve the UI of social.js:
#### iOS and Android
- descrease the size of the close button in a way that it does not overlay text in the webView
- use alloy's builtin popIn function to open the window (replaces the existing custom animation)

#### Android
- vertically center the progress bar inside the loading container
- remove the navigation bar (visible since 3.2.0 and the switch to heavyweight windows)
- use ```dp``` as the unit specifier

#### iOS
- increase the ```top``` property on iOS 7 to move the view below the status bar

Before and after screenshots both for Android and iOS:
![before_after_android](https://f.cloud.github.com/assets/4154538/1949757/f1848d18-811e-11e3-946f-20111a399a25.jpg)
![before_after_ios](https://f.cloud.github.com/assets/4154538/1949758/f1b1e02e-811e-11e3-80bc-77502cd89968.jpg)