Input Group Button Support

Added a new Extension to allow people to append buttons to inputs as well as icons.
Will add examples to the docs repo also. But this is what it looks like:

```
$builder
            ->add('keywords', 'text', [
                    'label' => false,
                    'attr' => array(
                        'input_group' => array(
                            'btn_append' => ['name' => 'search', 'type' => 'submit']
                        )
                    )
                ]);
```
Refactor

Renaming all btn references to button.
Merge pull request #287 from rdohms/input-group-buttons

Input Group Button Support
Improving isolation

Since symfony does not generate separate intances for each typeextension usage, adding the input
field name to the button storege is required to avoid conflicts.

Due to the nature of this process however, two forms with fields of the same name and with buttons appended
will trigger a conflict.
Merge pull request #292 from rdohms/input-button-group-improvement

Improving isolation
Adding ButtonTypeExtension which:
* fixes double 'type' in markup bug
* allows usage of BS-Button styled <a>-Tag
removing additional whitespaces in <button> markup
renaming button extensions new options for better clarity