Fix #430 Add linting and proper PEP8 testing

This change removes all old pep8 tests and instead has travis run
`flake8` after the unit tests. It will perform basic static analysis
(linting) on the entire `src/` tree and check for PEP8 conformance.

Contributors should run flake8 before committing their code for review:

    $ cd <where you checked out Freeseer>
    $ flake8 src

Also added a separate requirements file for developers. It will
also install the usual dependencies.

Change-Id: I0394f10cb7bb1291f2e993e94a4a2bfba9189495