Unify member names in heaps (#299)

The following renamings are made.

Public members:
- UnderFlowPriorityError -> PriorityUnderflowError
("underflow" is only one word)

Private members:
- bubble_up() -> bubbleUp()
- bubble_down() -> bubbleDown()
- second_child() -> secondChild()
- makeroot() -> makeRoot()
- relocate_last() -> relocateLast()
- data -> _data
- boxes -> _boxes

 INCLUDE_DIRECTORIES(

   ${PROJECT_SOURCE_DIR}

   ${PROJECT_BINARY_DIR}

 )

 LINK_DIRECTORIES(

   ${PROJECT_BINARY_DIR}/lemon

 )

 SET(DEMOS

   arg_parser_demo

   graph_to_eps_demo

   lgf_demo

 )

 FOREACH(DEMO_NAME ${DEMOS})

   ADD_EXECUTABLE(${DEMO_NAME} ${DEMO_NAME}.cc)

   TARGET_LINK_LIBRARIES(${DEMO_NAME} lemon)

 ENDFOREACH()