[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/load.py

@@ -1,37 +0,0 @@
-import bumblebee.module
-import multiprocessing
-import os
-
-def description():
-    return "Displays system load."
-
-def parameters():
-    return [
-        "load.warning: Warning threshold for the one-minute load average (defaults to 70% of the number of CPUs)",
-        "load.critical: Critical threshold for the one-minute load average (defaults 80% of the number of CPUs)"
-    ]
-
-class Module(bumblebee.module.Module):
-    def __init__(self, output, config):
-        super(Module, self).__init__(output, config)
-        self._cpus = 1
-        try:
-            self._cpus = multiprocessing.cpu_count()
-        except multiprocessing.NotImplementedError as e:
-            pass
-
-        output.add_callback(module=self.instance(), button=1, cmd="gnome-system-monitor")
-
-    def widgets(self):
-        self._load = os.getloadavg()
-
-        return bumblebee.output.Widget(self, "{:.02f}/{:.02f}/{:.02f}".format(
-            self._load[0], self._load[1], self._load[2]))
-
-    def warning(self, widget):
-        return self._load[0] > self._config.parameter("warning", self._cpus*0.7)
-
-    def critical(self, widget):
-        return self._load[0] > self._config.parameter("critical", self._cpus*0.8)
-
-# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4