Support inclusion or aggregation of other XML files

[fafux]

HI, I faced a problem in including xml files. I execute bin/t10_include_trees passing as first argument the path to examples/test_files/subtree_test.xml. Below the output:

terminate called after throwing an instance of 'BT::RuntimeError'
  what():  CheckTemperature is not a registered node, nor a Subtree
Aborted (core dumped)

To test I referred to the issue #17

[asasine]

The CheckTemperature and SayHello nodes were not registered with the factory for that example so I went ahead and added them. The output now looks like:

----------------
Sequence
   CheckStatus
      SequenceStar
         CheckBattery
         CheckTemperature
   OpenGripper
   CloseGripper
   SayStuff
      SequenceStar
         SaySomething
         SayHello
----------------
[ Battery: OK ]
[ Temperature: OK ]
GripperInterface::open
GripperInterface::close
Robot says: Hello World
Robot says: Hello World