SQL: put old tests back + some fixes to get them running

- removed meta keyword from has_table (was not used)
- added check for if_exists
- added table_exists back as alias for has_table
- added frame_query back as alias for read_sql (and deprecated it)
- changed way to delegate read_sql/table without using has_table for backwards compatibility (because this gave an error when uqing a mysql connection but not specifying the flavor)

Things that I had to change to the tests to get them running:
- changed MySQLdb to pymysql
- changed `sqlite3.OperationalError` to `pd.io.sql.DatabaseError` (in `test_tquery` and `test_uquery`)
- test_write_row_by_row are skipped for now (they depend on the issue of `execute` being able to handle cursor objects).
- in `test_execute` with pymysql converted the row values to python types (with numpy's `tolist()`), as pymysql errored on the numpy types.
DOC: release notes / v0.14.0 edits
DOC: Give KDE a section header

DOC: remove extra slice in hexbin example

DOC: move notes to look nicer
Merge pull request #7054 from TomAugspurger/visualization-cleanup

DOC: cleanup vis docs
BUG: fix reading multi-level index in python parser
Merge pull request #7029 from mcwitt/csv_mi_bug

BUG: fix reading multi-index data in python parser
DOC: fix formatting on plot formattting

DOC: add KDE to other plots

more bits of foramtting
Merge pull request #7062 from TomAugspurger/visualization-doc3

DOC: One more cleanup on visualization.rst
BUG: PEBKAC bug in Float64Index
TST: add test for multiple element Float64Index containment
FIX: execute() works on a cursor.
Merge pull request #6987 from jorisvandenbossche/sql-legacy-tests

SQL: bunch of fixes based on the old tests
Merge pull request #7068 from cpcloud/float64-index-bug

BUG: PEBKAC bug in Float64Index
ENH: use size instead of cythonized count for fallback cases
Added text to the HDFStore.put() method indicating dropna is available and settable via option, as for append()
BUG: df.boxplot fails to use existing axis/subplot (#3578)

axes needs to be a plt.Axes object

use ax get_axes() to set axes

Add tests for use of existing axis

Add test for when by is None

Add fix to the release notes
BUG: Fix Xl(sx|wt)Writer always using `date_format`.

Since `datetime.datetime` derives from `datetime.date` the second
`if`-clause will always trigger, leading to the `num_format_str` being
overwritten by the `date_format`.
Merge pull request #7075 from filmor/patch-1

Fix Xl(sx|wt)Writer always using date_format even if a datetime is supplied.
SQL: resolve legacy mode + deprecate mysql flavor (GH6900)

- removed warning for not using sqlalchemy (as sqlite DBAPI connection is fully supported, no warning needed)
- added deprecation warning for 'mysql' flavor
- removed necessity of providing flavor kwarg (no warning if not provided, assumed to be sqlite3)
- removed `flavor` kwarg from execute and read_sql_query (because a) are not database specific functions + b) would only be needed for mysql, but this will is deprecated so no need to introduce it)
- updated the tests to reflect this
- updated docs and docstrings to reflect that sqlite3 is the only but fully supported DBAPI connection (no 'legacy')
Merge pull request #7064 from davidljung/master

Added dropna to docstring for HDFStore.put()