scripts: (temperature) new script.

This script is based on [scripts/cpu_usage](https://github.com/vivien/i3blocks/blob/master/scripts/cpu_usage) and the [Wiki's example](https://github.com/vivien/i3blocks/wiki#temperature). I dropped the `+` for positive temperatures.

The script itself only prints the temperature in celsius degrees. The warning temperature, the critical temperature as well as the sensor chip can be forwarded to the script.

Example:

![temperature](https://cloud.githubusercontent.com/assets/2742231/3316342/c49c406c-f701-11e3-8345-5cd39fcf529f.png)

```text
[cpu]
command=$BLOCK_LIBEXEC/cpu_usage
interval=10
min_width=CPU: 99.99%

[cpu-temp-pre]
command=echo "("
separator_block_width=0

[cpu-temp]
command=$BLOCK_LIBEXEC/temperature
interval=10
separator_block_width=0

[cpu-temp-post]
command=echo ")"
```

The "coating" (e.g. `Temp: `) can be done outside of the script, which gives more flexibility to the user.