Cleaned unused imports, added ability to set custom font

I cleaned unused imports for the sake of getting rid of IDE warnings.

Other than that, I added the ability to set a new alert font without having to create an entirely new theme or edit an existing one. 

i.e I like the bootstrap colors, but the font conflicts with the rest of my apps ui, so instead of rebuilding the theme  I can simply call this function in my code
```java
     public static void displayAlert(final Font font, final String message,
			final AlertifyType type, final int time) {
		Alertify.show(new AlertifyBuilder().font(font).type(type).text(message)
				.autoClose(time).build());

	}
```

Which is a lot easier when trying to stream line a bunch of alerts where you want the font to be different per alert.