[core] restructure to allow PIP packaging

OK - so I have to admit I *hate* the fact that PIP seems to require a
subdirectory named like the library.

But since the PIP package is something really nifty to have (thanks to
@tony again!!!), I updated the codebase to hopefully conform with what
PIP expects. Testruns so far look promising...

bumblebee_status/modules/core/disk.py

@@ -0,0 +1,65 @@
+# pylint: disable=C0111,R0903
+
+"""Shows free diskspace, total diskspace and the percentage of free disk space.
+
+Parameters:
+    * disk.warning: Warning threshold in % of disk space (defaults to 80%)
+    * disk.critical: Critical threshold in % of disk space (defaults ot 90%)
+    * disk.path: Path to calculate disk usage from (defaults to /)
+    * disk.open: Which application / file manager to launch (default xdg-open)
+    * disk.format: Format string, tags {path}, {used}, {left}, {size} and {percent} (defaults to '{path} {used}/{size} ({percent:05.02f}%)')
+"""
+
+import os
+
+import core.module
+import core.widget
+import core.input
+
+import util.format
+
+
+class Module(core.module.Module):
+    def __init__(self, config, theme):
+        super().__init__(config, theme, core.widget.Widget(self.diskspace))
+
+        self._path = self.parameter("path", "/")
+        self._format = self.parameter("format", "{used}/{size} ({percent:05.02f}%)")
+
+        self._used = 0
+        self._left = 0
+        self._size = 0
+        self._percent = 0
+
+        core.input.register(
+            self,
+            button=core.input.LEFT_MOUSE,
+            cmd="{} {}".format(self.parameter("open", "xdg-open"), self._path),
+        )
+
+    def diskspace(self, widget):
+        used_str = util.format.byte(self._used)
+        size_str = util.format.byte(self._size)
+        left_str = util.format.byte(self._left)
+        percent_str = self._percent
+
+        return self._format.format(
+            path=self._path,
+            used=used_str,
+            left=left_str,
+            size=size_str,
+            percent=percent_str,
+        )
+
+    def update(self):
+        st = os.statvfs(self._path)
+        self._size = st.f_blocks * st.f_frsize
+        self._used = (st.f_blocks - st.f_bfree) * st.f_frsize
+        self._left = self._size - self._used
+        self._percent = 100.0 * self._used / self._size
+
+    def state(self, widget):
+        return self.threshold_state(self._percent, 80, 90)
+
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4