add "throttle" and "bandwidth" options (RhBug:1045737)
util: function to log exceptions' tracebacks.

Also log the different IOErrors causing early exit.
cli: --cacheonly beats expired repos. (RhBug:1048468)

document it, test it.
progress display: end(size=None) should never fail
report mirror failures (RhBug:1051554)
drpm: add config options + docs, load updateinfo.xml
drpm: download delta packages if possible
get_package_target(): support the "donecb" callback
drpm: spawn delta rebuild jobs, report failures
drpm: display delta rebuilds as they finish
drpm: fall back to rpm download on drpm errors
test_drpm: Add some tests before calling base.download_packages()