Improved likeness to Apple's blurring effect, and some usability tools

The following makes the blur effect fit the appearance of the native controls:
* I've dropped the default 0.25 alpha value for the tintColor, because since 7.0.3 Apple is also considering it for the blur effect.
* I've added a monochrome mode: when you set UIToolBar or UINavigationBar tintColor, the color of the blur effect becomes monochromatic in Apple's implementation.

The following improve the usability of the app:
* I've added min/max brightness levels, so when adding controls of known color to the FXBlurView, you can insure there won't be "brightness collusion" between the background and the controls. (For my case, I'm using white text, and I really needed to ensure the brightness won't go over 0.9, so the text will always be readable).