Update debug levels and message in OpenNI device

Change some debugging levels to SEVERE, since they occur when the OpenNI
camera is unusable. Added some debug info to a couple of asserts.

Also, removed a check of how many devices are connected. This was added
for debug purposes and isn't needed.

Added doxygen comment for the return value os
OpenNIDevice::isInitialized
Remove unneeded libs from OpenNI UnitTests

A couple of libs don't need to be explicitly linked into the OpenNI unit
tests.
Move initialization to doInitialize in OpenNIScaffold

Move the sdk initialization into the thread doInitialize. Also, during
registerDevice, start the thread if it hasn't been started, and wait for
it to be initialized before adding the device. This is required becuase
the SDK must be initialized before a new device can be added.
reset datagroup entry in OpenNI if no data

If no data is read from the OpenNI camera, reset the entry in the data
group to indicate the data is invalid. Note, if OpenNI's readFrame does
not return STATUS_OK, this indicates something is wrong, not that there
is no data.
Add NimbleDevice to the devices doxygen page