Moved sources into main folder

Hi! Thanks for writing this dart package! I tried to consume it through the pub functionality of dart, but it couldn't find the pubspec.yaml since it was located in a deeper folder hierachy.
So I forked your project and moved the files around. If there is a better way of doing this, please let me know. If not, you might want to consider including this change, so that others can easily consume thid package, too.