add cpp code for [[ method
add readFrame.cpp
add subsetting by bitvector to cpp code
j subsetting by other types of vector
1. simplify bitvector parsing 2.construct List instead of data.frame
add i parsing in C api
use RcppArmadillo to speed up matrix subsetting. #33
use CPP version of [[ by default. close #33
wrap bit vector operation in cpp
optionally disable validity check in Subset method to speed it up