Proper position for placeholder

I modify some codes in `- (CGRect)placeholderRectForBounds:(CGRect)bounds`, which returns proper CGRect value for drawing.

Here are some screenshots for comparison, you can see the different clearly.

### Before
![before](https://f.cloud.github.com/assets/1225437/1480951/b72b7a04-46be-11e3-9fe1-71dd7b034f86.png)

### After
![after](https://f.cloud.github.com/assets/1225437/1480957/d2041bba-46be-11e3-877a-43107bb0cd68.png)

### Typing some words
![typing](https://f.cloud.github.com/assets/1225437/1480958/db040626-46be-11e3-9961-e66cd90dc4f2.png)