Move requirements.txt to it's proper root-level position.

--HG--
rename : develop/requirements.txt => requirements.txt
Better reorganize the project structure along the lines of http://www.jeffknupp.com/blog/2013/08/16/open-sourcing-a-python-project-the-right-way/.

--HG--
rename : convocore/cli.py => convocore/cli/cli.py
rename : convocore/error.py => convocore/core/exceptions.py
rename : convocore/utility.py => convocore/core/utility.py
Seems a bit much to name a file 'exceptions' when there's only the one exception inside.

--HG--
rename : convocore/core/exceptions.py => convocore/core/exception.py