Added support for remote hosts, fixed "performance" leak

[Sickboy78]

Added the support to get system stats from remote hosts.
Since commands to get stats differ from system to system, I made the command and also the split and replace commands configurable.

There also was a problem with the interval timers in the node_helper.js. With every page request new timers where created but never stopped/deleted. This resulted in increasing faster updates and also increasing performance and memory consumption until raspberry crashes.
I fixed this by moving the interval timers to the client side, so they are resetted with each page request.

[BenRoe]

Wow nice feature, but a lot of code changes. I have to test this first.
Btw can you split the configuration table in the readme into two sections (basic & advance/remote).

[Sickboy78]

Ok, I'm gonna have a look at it.

[Sickboy78]

Hi Ben, I splitted the configuration into basic and advanced config values.
But I had to remove the config.unit value, since the commands are no longer hard coded in this version, but are also configuration values. I added the commands for the different units as example values in the readme.
If you want to keep the units or keep things simple, I can understand that. So if you dont want to merge the changes, please tell me, then I will rename the fork (e.g. MMM-SystemStatsRemote) and keep it as a separate module.
Happy Easter!

[BenRoe]

Thank you @Sickboy78 currently i am a bit busy. First impression looks good.
Have setup a mm and test it.