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.
One more bug fix

The for loop used `psi5_groupby` instead of `groupby`.
One more README update

Need min_multimap=0 when calling get_psis.
Merge pull request #1 from cdeboever3/master

README update and bug fixes