Added hover props and fixed style bug.

[mmrtnz]

These changes and bugs were added/discovered as a result of refactoring toolbar.

• Added hover prop to the following components.
  • FontIcon and SvgIcon:
    • hoverStyle: Inline-style applied when the component is being hovered.
  • DropDownMenu
    • styleIconHover: Inline-style applied to the drop down arrow icon when it is being hovered.
  • DropDownIcon
    • hoverStyle: Inline-style applied to the icon when it is being hovered.
    • Additional Props
      • iconStyle: Inline-style applied to the icon of the DropDownIcon.
      • iconClassName: CSS classname added to the icon of the DropDownIcon for additional styling.
• Fixed style bug
  • If a specific style prop is undefined (i.e. hoverStyle) and is passed as an argument to StylePropable's mergeAndPrefix() function, it defaults to the components main style prop this.props.style. If-statements have been added where appropriate to catch this bug.

@hai-cea what are your thoughts regarding this hoverStyle prop? Should the user be responsible for handling pseudo-selectors when overriding mui component inline-styles?