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. _Or maybe, there is a better way to do this_.

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`