Remove reliance on linux / unix tail command.

[AndyTaylorTweet]

Replaced the reliance on 'tail' in the admin module.
Source PHP from here: http://tekkie.flashbit.net/php/tail-functionality-in-php

This should provide a like for like drop in for tail so that Windows server users can still get the log file displayed in the admin module.

[AndyTaylorTweet]

This PR relates to this;
https://community.openenergymonitor.org/t/for-windows-users/1958

[Paul-Reed]

Thanks Andy - a nice update which Windows users will appreciate.
Works fine on Linux, and once merged hopefully will be tested by a Windows user.
Hate to ask Willems @joyrider3774, but...

Paul

[joyrider3774]

@Paul-Reed works perfect on windows
@AndyTaylorTweet thanks for the modifcation modifcation it removes the need for gnu utils forks well at least there, don't know if there are other places that uses these utils (i know cat is used but only on the emonpi section so that's okay as this is for raspberry pi or at least i think so since i don't see the emonpi admin menu).