ENH: add method='dense' to rank
Addresses #6333.

(Let's see how it goes on Travis; last time there was some strange behaviour which I think turned out to be because of an uninitialized variable which was behaving differently on my system than on the buildbots.)

looks good
pls add a release note entry (and same in v0.14 if u want)

then good. 2 go
 are their frame tests at all? 
 Yeah, `test_rank_2d_tie_methods` loops over every rank method and checks the frame behaviour.  Useful, too-- one of my early versions was resetting the count at the wrong place, and so it was ranking the whole flattened frame rather than each column.
 ahh ok makes sense
 perfect pls just add release notes and can merge
 So does this go under "Improvements to existing features" for 0.14.0?
 that works
 It is my considered opinion that git is sufficiently complicated that having a tutorial reference open while using it is useless, because there is always some state I'm in which prevents the command from working as it's supposed to.  And this is a fact which beginners have no way to know until they've done it and made things worse.

Right now the last the last three log events in my local branch add_dense_rank_v5 are (1) the original commit, (2) 7ec7662's commit, and (3) a merge which was introduced when trying to follow instructions to avoid the "Updates were rejected because a pushed branch tip is behind its remote" error.
 hmmm...I avoid merging (except when actually merging other people commits)...

my workflow is something like this:

update master 

```
git checkout master
git pull --rebase
```

```
git checkout branch
git pull --rebase
```

fix conflicts (if needed)

```
git add --all
git rebase --continue
```

add commits

rebase when needed to reorder/squash

```
git rebase -i <commit before this branch>
git push myfork thisbranch -f
```
 you can just delete that merge commit FYI (I am going to remove it when I merge you in a moment anyhow)
 merged via https://github.com/pydata/pandas/commit/61b141bc934c7e234f9675234a578f043f3bbdd1
 thanks as always!
