Fix various compiler warnings and minor bugs

Fix various compiler warnings emitted by gcc-4.6.3, some of which indicate bugs
    
    * Mark boost and python headers as system headers to avoid a pile of
      (mostly boost) warnings in headers we don't control.
    
    * Remove assertion about the return value of memcpy, because memcpy
      unconditionally returns the dest buffer regardless of success.
    
    * Avoid unused variable warnings
      - When building without hexer
      - Due to variables only used in asserts
      - Unused endianess variable
    
    * Fix bug in las writer when reading uint8_t intensity from buffer,
      which would have accessed a bogus pointer and caused a segfault.

Also fix minor memory leak of stage factories.