[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/disk.py

@@ -1,40 +0,0 @@
-import os
-import bumblebee.util
-import bumblebee.module
-
-def description():
-    return "Shows free diskspace, total diskspace and the percentage of free disk space."
-
-def parameters():
-    return [
-        "disk.warning: Warning threshold in % (defaults to 80%)",
-        "disk.critical: Critical threshold in % (defaults to 90%)"
-    ]
-
-class Module(bumblebee.module.Module):
-    def __init__(self, output, config):
-        super(Module, self).__init__(output, config)
-        self._path = self._config.parameter("path", "/")
-
-        output.add_callback(module=self.instance(), button=1, cmd="nautilus {}".format(self._path))
-
-    def widgets(self):
-        st = os.statvfs(self._path)
-
-        self._size = st.f_frsize*st.f_blocks
-        self._used = self._size - st.f_frsize*st.f_bavail
-        self._perc = 100.0*self._used/self._size
-
-        return bumblebee.output.Widget(self,
-            "{} {}/{} ({:05.02f}%)".format(self._path,
-            bumblebee.util.bytefmt(self._used),
-            bumblebee.util.bytefmt(self._size), self._perc)
-        )
-
-    def warning(self, widget):
-        return self._perc > self._config.parameter("warning", 80)
-
-    def critical(self, widget):
-        return self._perc > self._config.parameter("critical", 90)
-
-# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4