Add and attribute to disable the inner shadow

The official guideline does not specify if a inner shadow has to be applied on the Floating Action Button.
Some screens have one:
![Shadow](http://material-design.storage.googleapis.com/publish/v_1/quantumexternal/0Bx4BSt6jniD7TGVDMFY1NkRZaEk/style_color_uiapplication_accent1.png)
Some don't
![No shadow](http://material-design.storage.googleapis.com/publish/v_1/quantumexternal/0Bx4BSt6jniD7Slc2dnJPQ0ViMzA/patterns_actions_fab1.png)

In some cases (with a dark theme for example) the inner shadow is not that pretty.

This pull request just add an attribute to disable the inner shadow.