Fix for placeholder alignment issue 

Now the placeholder text aligns in the middle of the
RPFloatingPlaceholderTextField just like as in the the default
UITextField irrespective of the height of the box

Before:
![before](https://f.cloud.github.com/assets/1040583/1435775/ed721676-4147-11e3-9f9d-302c6b8eceb3.png)

After : 
![after](https://f.cloud.github.com/assets/1040583/1435779/005103d8-4148-11e3-9406-4ce04aa8890b.png)