[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/memory.py

@@ -1,38 +0,0 @@
-import psutil
-import bumblebee.module
-import bumblebee.util
-
-def description():
-    return "Shows available RAM, total amount of RAM and the percentage of available RAM."
-
-def parameters():
-    return [
-        "memory.warning: Warning threshold in % of memory used (defaults to 80%)",
-        "memory.critical: Critical threshold in % of memory used (defaults to 90%)",
-    ]
-
-class Module(bumblebee.module.Module):
-    def __init__(self, output, config):
-        super(Module, self).__init__(output, config)
-        self._mem = psutil.virtual_memory()
-
-        output.add_callback(module=self.instance(), button=1, cmd="gnome-system-monitor")
-
-    def widgets(self):
-        self._mem = psutil.virtual_memory()
-
-        used = self._mem.total - self._mem.available
-
-        return bumblebee.output.Widget(self, "{}/{} ({:05.02f}%)".format(
-            bumblebee.util.bytefmt(used),
-            bumblebee.util.bytefmt(self._mem.total),
-            self._mem.percent)
-        )
-
-    def warning(self, widget):
-        return self._mem.percent > self._config.parameter("warning", 80)
-
-    def critical(self, widget):
-        return self._mem.percent > self._config.parameter("critical", 90)
-
-# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4