yum: Fix apt-dater-host with newlines in yum

apt-dater-host calls yum, which asks urllib(!) for its idea of the
current terminal width, which causes yum to break the output into
"human-reable" one.

This commit wraps yum calls into call_yum, which, in turn, appends lines
beginning with ^\s+ to the previous one to ease parsing.
Merge pull request #4 from yath/yum-newline-fix

yum: Fix apt-dater-host with newlines in yum (fixing DE-IBH/apt-dater#15).