Wrapping radio_button tag, new feature for #check_box and refactored some code

Hi Stephen,

i made some new improvements on the gem.

* New ```#radio_button``` tag, it works like the ```#check_box```
```<%= f.radio_button :foo, 'bar', label: 'Bar' %>``` 

* New ```:inline``` option to ```#check_box``` and ```#radio_button``` methods
```<%= f.radio_button :foo, 'bar', label: 'Bar', inline: true %>```

* And i made some changes on the code, take a look :smile: