fixed error message for irregal 2nd argument
implement get_sumPMI
added comment for get_sumPMI
added prefix 'self.' for every function in module ntcir
deleted magic number
calcPMISimilarity can consider vector's value
[bugfix]rejection rate is fixed into acceptance rate
added comment for arr.slice!
[feature] implemented calc_ap
[expansion]pmi between same words should be higher values.