Support the popular pip editable installation mode.
To work-around a problem in setuptools' develop target, generalize
'install_dir' to say all packages are in 'src' (for more info, see:
https://github.com/pypa/pip/issues/126).
Remove pre-generated bin/tito-dev script.
Ensure build deps are consistent across the spec file and test harness.
Note: http://fedoraproject.org/wiki/HOWTOFindMissingBuildRequires
says that some packages should not be BuildRequires because
"they would occur too often" and "are considered the minimum build
environment." However, Docker changes the scope of that assumption
to keep docker images small.
