Add support for linux with upower

When pmset is not available try using upower which works on linux
Add linux support on percentage display

Again use upower to display percentage
Change icons to things that work

For some reason, the default icons are not available in my font.
Change them to things that are available
Bring back icon defaults for OS X
Refactoring functions to a helper file
DRYing the code
Change plugin name to `tmux-battery`
Mention contributors in the readme