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.
type/Default: don't depend on external function GET
type: move generic functions to base class
move collectd_flush function to base class
type/base: set default values for rrd_title and rrd_format
rename ds_names and dsname to legend

This had nothing to do with an RRD data source.
type/base: show tinstances not defined in $this->order in graph

Previously if $this->order was defined in a graph, only type instances
that were defined in $this->order showed up in the graph. Now it merges
$this->order and the type instances.
type/base: always use rainbow_colors and overwrite them afterwards

Since the previous commit [type/base: show tinstances not defined in
$this->order in graph] all type instances that are not defined in the
plugin, are colored black. This way you always have nice colors.
type/base: set default title to "Plugin Type (PluginInstance) (Category)"
graph.php: use JSON plugins instead of including PHP plugin files

A couple of big changes here. A lot of logic moved to graph.php.

The PHP plugin files have been rewritten to JSON. In these JSON files
*everything* is optional. Also *NOT* having a JSON plugin file won't
block you from having a graph. The JSON will just make the graphs
prettier (by having a title, y-axis title, legend, colors, etc..).

The Collectd types.db file is parsed and used to determine RRD content.
When things are not defined in the JSON it will fallback to a default.
plugin/processes: add wait, idle

Closes: https://github.com/pommi/CGP/issues/69
plugin/memory: add cache

Closes: https://github.com/pommi/CGP/issues/71
inc/html.inc.php: fix canvas graphs

Include graph.php instead of non-existing plugin/$plugin.php.
plugin/uptime: fix legend format (.1lf -> %.1lf)

Closes: #72
plugin/libvirt: show (v)CPU time in milliseconds
Add base option (1GB=1024Mo)
Add base on all df plugins
detail.php: fix canvas graphs

Include graph.php instead of non-existing plugin/$plugin.php.

Closes #75