Format item values

Format item values in "Overview: Items" and "Latest data" page to make easy to read.

* Add units
* Add metric prefixes
* Convert unix time to human readable date
* Convert uptime to human readable time

It's based on Zabbix's implementation.
If a value has no unit or the value type is string, don't format it.

![formated-item-values](https://cloud.githubusercontent.com/assets/135104/4970214/dc8b0b04-687b-11e4-997b-e342889dbd3f.png)