Adding support for batteries without energy_now and energy_full.

If these values are not available it will try to use charge_now and
current_now instead.
Fixed for OS X Mavericks
t/01-tmux.t improved
Fallback to /usr/bin/vm_stat on OS X Mavericks (because /usr/bin/top output is way too incomplete)
Merge pull request #18 from jmatth/extra_bat_support

Adding support for batteries without energy_now and energy_full.
Documentation update
Added the --datfile option, as suggested in issue #19
Version bump