Implemented setTaskPriority() and getTaskPriority() in Task HAL API
Removed all other unimplemented functions in HAL and removed spawnTask()
Replaced instances of pthread_t* with TASK typedef
Removed unused HAL error constants and removed commented-out classes and functions in wpilibj's HALLibrary
Changed Task class API to match the construction semantics of a std::thread
Change-Id: I3bc951a3da90d24c5589fae4d1ca2bb60225c873