overview: only calculate cores when config option showload is enabled

Closes: https://github.com/pommi/CGP/issues/60
fix: return operator prevents us from seeing a graph
Merge pull request #61 from young-druid/patch-1

fix: return operator prevents us from seeing a graph
overview: show used memory percentage
overview: disable showmem by default, add percent sign

And change all spaces to tabs.
plugin/df: fix support for collectd 4

Support for collectd 4 was broken since commit:
v0.4.1-52-g0c50eff [Plugin/df: Add support for inode graphs]

Closes: https://github.com/pommi/CGP/issues/64
plugin/memory: add support for osx/bsd

Closes: https://github.com/pommi/CGP/issues/67
plugin/memory: add osx/bsd ds_names to order, set color of active to ff00ff

Closes: https://github.com/pommi/CGP/issues/67
inc/collectd: consistently return false instead of an empty array or false

This means we have to check the return value of these functions for
example before doing a foreach. We can't just do a foreach and assume the
return value of a function is an array, while it can be false and results
in a PHP warning.

Closes: https://github.com/pommi/CGP/pull/68
inc: mv graphs_from_plugin, build_url functions to html.inc.php

They generate html.
plugin: add bcache plugin

This bcache plugin generates graphs for the collectd-bcache script used
in combination with the Collectd Exec plugin.

Reference: https://github.com/pommi/collectd-bcache