ENH: series rank has a percentage rank option
closes #5971
This pull allows people to compute percentage ranks in cython for a series. I found myself computing this all the time and it will make this calculation much faster.  

can you add a vbench for both cases (the original and new one?)
 any thoughts on 'percentage' vs. say 'per'? @jorisvandenbossche  @y-p ?
 I prefer 'pct' to 'per' @jorisvandenbossche @y-p 
 @MichaelWS so change to that..
 done
 also, I have never worked with vbench before, so I am not sure if I am doing what is intended
 add a benchmark in vb_suite/groupby.py.....just copy-paste what you want to do

`./test.perf.sh -b prior_commit -t last hash_of_yours`
 I am getting some odd errors. I will have to do some more research later today on it.

sqlalchemy.exc.IntegrityError: (IntegrityError) column checksum is not unique u'INSERT INTO benchmarks (checksum, name, description) VALUES (?, ?, ?)' ('ea1993ef61c3cc4e871d2cce3c5d983c', 'eval_frame_chained_cmp_python', None)
 this looks fine
can u rebase and move release notes to 0.14
 easy enough.  That is done.
 can you reset the release.rst to master HEAD then add your change again. Their were some formatting updates which you are reverting....this should only be your 1 line change
 thanks @MichaelWS !
 Thanks @jreback 
 @MichaelWS I think we need to have this `pct` option on DataFrame for consistency as well. Can you add?

issue is here: https://github.com/pydata/pandas/issues/6717
