Dear Maintainer,
I get this error when running batalert:
```
$ batalert
Traceback (most recent call last):
5: from /usr/bin/batalert:23:in `<main>'
4: from /usr/bin/batalert:23:in `load'
3: from /usr/share/rubygems-integration/all/gems/batalert-0.4.0/exe/batalert:5:in `<top (required)>'
2: from /usr/share/rubygems-integration/all/gems/batalert-0.4.0/lib/batalert.rb:14:in `main'
1: from /usr/share/rubygems-integration/all/gems/batalert-0.4.0/lib/batalert.rb:14:in `open'/usr/share/rubygems-integration/all/gems/batalert-0.4.0/lib/batalert.rb:14:in `initialize': No such file or directory @ rb_sysopen - /sys/class/power_supply/BAT0/capacity (Errno::ENOENT)
```
This appears to be caused by batalert making an incorrect assumption
about the battery name:
```
$ ls /sys/class/power_supply/
ACAD BAT1
```
I expected batalert to either correctly detect the device battery name or at least to provide a flag for setting it.