[core] Refactor engine

This is going to be a bit more comprehensive than anticipated. In order
to cleanly refactor the core and the engine, basically start from
scratch with the implementation.

Goals:

* Test coverage
* Maintain backwards compatibility with module interface as much as
  possible (but still make modules easier to code)
* Simplicity

see #23

bumblebee/modules/cpu.py

@@ -1,30 +0,0 @@
-import bumblebee.module
-import psutil
-
-def description():
-    return "Displays CPU utilization across all CPUs."
-
-def parameters():
-    return [
-        "cpu.warning: Warning threshold in % of disk usage (defaults to 70%)",
-        "cpu.critical: Critical threshold in % of disk usage (defaults to 80%)",
-    ]
-
-class Module(bumblebee.module.Module):
-    def __init__(self, output, config):
-        super(Module, self).__init__(output, config)
-        self._perc = psutil.cpu_percent(percpu=False)
-
-        output.add_callback(module=self.instance(), button=1, cmd="gnome-system-monitor")
-
-    def widgets(self):
-        self._perc = psutil.cpu_percent(percpu=False)
-        return bumblebee.output.Widget(self, "{:05.02f}%".format(self._perc))
-
-    def warning(self, widget):
-        return self._perc > self._config.parameter("warning", 70)
-
-    def critical(self, widget):
-        return self._perc > self._config.parameter("critical", 80)
-
-# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4