added new function of SpansReg

[shewer]

擴展 Spans

-- return Spans 
cand:spans()
segment:spans()
composition:spans()

-- spans api 
spans:add_vertex( size_t vertex)  
spans:clear() --  vertices = {}
spans.vertices -- return **size_t** of  lua_table
spans.vertices = { 5, 4, 2 , "3 " ,4 }  -- vertices = { 2, 3, 4, 5}

[hchunhui]

@ksqsf 可以请你帮忙review一下吗？