Fixed README

In the code example at the end, the last_bp_intron for the first splice
site was 100 but should be 180 to match the example above.
Bug fixes

psi3 was being calculated by grouping on first_bp_intron but it should
be last_bp_intron. This returns correct results for the README example.
The call to sj.set_index needs to have drop=False or the chrom column is
lost after calculating psi5 and psi3 won't work.