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.