Added display of commit age

Now shows age of commit (e.g. "2 months ago") in the modeline, and full
details of the commit in the minibuffer while navigating through old
commits.

Added new customisation 'git-timemachine-show-minibuffer-details' to
control the display in the minibuffer in case it is distracting.
Added minibuffer echo customisation to README
Merge branch 'hickinbottoms-feature/show-commit-age'
Drop dependency on cl-lib.
Bumped up version.