Feature/show commit age

Enhanced display in modeline to include commit age (in 'relative' git terms) next to the commit index, e.g.:

`timemachine:git-timemachine.el@12345678 (43/43 10 minutes ago)`

Additionally, the minibuffer will show full commit details:

`commit f5c4e6f5e05eab2b9b7cdc778a8048129ab887bc Wed Nov 5 22:49:58 2014 +0000 (10 minutes ago)`

Just in case the minibuffer thing is distracting, a new customisation `git-timemachine-show-minibuffer-details` has been added, which defaults to *t*.

If you've any suggestions for modifications to this then please just let me know.