Sensor Objects

This includes all the sensor objects including a few
implementations, including dbus and sysfs sensors.

Change-Id: I9897c79f9fd463f00f0e02aeb6c32ffa89635dbe
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/notimpl/README b/notimpl/README
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+These are convenience objects that let one create a read-only sensor or a
+write-only object in whatever scenario.
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