Integrate pyflakes-enhanced AST into coala

The idea, here, is to integrate pyflakes-enhanced AST into coala as a metabear which can then be used to develop various plugins. The second part of the project involves redesigning flake8 plugins flake8-future-import and flake8-builtins in such a way that they use pyflakes-enhanced AST over python AST. Finally, a wrapper is to be created which supplies a python AST to flake8 plugins so that they work as it is.