CLN: move plotting funcs to pd.plotting
Rebased in #16005

---

- [x] closes #12548
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

cleanup `tools/plotting` as `plotting` subpackage. Because the differences are being huge, would like to split 3 part if base direction is OK:

~~1. Move plotting related tests under `pandas/tests/plotting` splitting to `Series`, `DataFrame`, etc. (0.18.2)~~ (#13621)
2. Create `pd.plotting` and move related files (0.20.0) 
3. Split / refactor `pd.plotting` to use small utility functions / property control classes. Add tests. (0.20.0)

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/13579?src=pr) is 85.68% (diff: 69.35%)
> Merging [#13579](https://codecov.io/gh/pandas-dev/pandas/pull/13579?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #13579   diff @@
==========================================
  Files           145        154     +9   
  Lines         51419      51464    +45   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          44057      44098    +41   
- Misses         7362       7366     +4   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [97fd744...1c1efe1](https://codecov.io/gh/pandas-dev/pandas/compare/97fd7447ff54d8f964d057dbaad2b5caa362e24d...1c1efe190667f1d868c1ab95a12d754458324d3b?src=pr) @sinhrks Generally sounds like a good plan!
 +1 overall. @sinhrks will this break old code? Does `pandas.tools.plotting` just point to `pandas.plotting`? I looked in the diff, but could have missed it.
 @sinhrks Can you rebase this? Further it is mainly docs that is still needed?
 @sinhrks as usual, certainly can include in 0.19.0, just trying to clear the decks.
 @sinhrks can you update 
 @sinhrks do you time to update this? @sinhrks Do you have time to update this? I still think this is a useful refactor. I can also do a rebase to get it in. Once updated to the latest master. Let me add release note later. Thx, updated based on the comments.

- tests are moved to ``pandas/plotting/tests``.
- ``pd.scatter_matrix`` has been deprecated, in favor of ``pd.plotting.scatter_matrix``.
- added tests to check warnings are properly raised. really just some minor comments. ok with merging, then fixing after if it is easier (one test failure on locale on travis, not sure why that is). @sinhrks seems really close...can you rebase I rebased this one, but pushed to a new PR: https://github.com/pandas-dev/pandas/pull/16005

(this PR predates from when you can push to PR's branch)