* Fix battery plugin when acpi writes to stderr
* Make stderr redirection in battery plugin more idiomatic

The $OSTYPE variable is set at ZSH compile time and can be safely used
to determine the OS of the system. e.g. darwin (os x)

Instantly after removing the charger, I have found that the "AvgTimeToEmpty" value can swing to rediculous values. Apple's current claim is that the max battery life is 12 hours, so any value larger than this is considered a "time is being calculated" value.

It is especially useful for themes, this function is a port of the code written by Steve Losh
and made available on [this blog entry](http://stevelosh.com/blog/2010/02/my-extravagant-zsh-prompt/#my-right-prompt-battery-capacity)

Define empty functions instead of none at all if we can't figure out the
platform.

Separate the battery_pct_remaining data into it's own function so that it can be obtained even if the battery is connected.

linux battery level now refreshes after every command