Fixed placeholder position for SSTextField on iOS 7

In iOS7 all methods like like drawInRect:withFont: are deprecated and give the wrong text appearance (always aligned to top). Rect should be adopted for vertically centering text. 

Example of incorrect placeholder position:
![Placeholder](http://habrastorage.org/storage3/3e3/aa2/b48/3e3aa2b482d57c3c46c216adbd5b83cc.jpg)

Added rect adjustments for iOS 7 as adviced [here](http://stackoverflow.com/a/3396065/2797141):
```objective-c
CGRectInset(rect, 0, (rect.size.height - self.font.lineHeight) / 2.0);
```