Clean up flake8 warnings

Run 'make lint' and fix all warnings and errors.

Signed-off-by: Wei Liu <liuw@liuw.name>
requirements.txt: more requirements

Introduce:
 * Sphinx for building documentation
 * Nose and coverage for testing
 * flake8 for checking coding style

Signed-off-by: Wei Liu <liuw@liuw.name>
gitignore: ignore coverage file

Ignore .coverage which is generated by code coverage test.

Signed-off-by: Wei Liu <liuw@liuw.name>
Makefile: mark all targets as PHONY targets

... as none of them produces a real file.

Signed-off-by: Wei Liu <liuw@liuw.name>
Merge pull request #78 from liuw/misc-clean-up