Compute mean percentage for multiple batteries.

If a laptop has multiple batteries, compute the mean percentage if the
user asks for 'all' instead of a specific battery.
Use strncmp() when checking battery name.
Update docs for battery_bar and battery_percent.
Address PR comments.

In particular:
- Don't call get_battery_perct() recursively.
- Use define for length of string holding battery name.
- We can use strcpy() since bat is zero-terminated.
Merge pull request #44 from ldx/allbattperct

Compute mean percentage for multiple batteries.