Fixed issue #15 - Attribute name clashes

As per issue #15 and #14, I've renamed all attributes to have a prefix `flatui_`.

I'm not sure about other complications this will bring later because I only use the theme via xml. Please do fix this if it exists in the programmatic way as well.

Also, please read README.md for the changes that has to be made in existing app.

tl;dr you need to change the `flatui:theme` to `flatui:flatui_theme`