Qt5 supported (#252)

* Qt5 supported

Since support added, seems important enough to list (order important? ok as is?); while synonyms(?) may not be important to list: :qt_pyqt4=>"Qt4Agg", :qt_pyqt5=>"Qt5Agg", :qt_pyside=>"Qt4Agg"

[skip ci]

* And Tk

Interesting that Tk seems a fist choice (or not; with different ordered list for each platform). Hopefully good enough link:
"changed the title from Tkinter library conflict on MacOS X to Tk library conflict on MacOS X on 13 Jul"

[skip ci]

Author: PallHaraldsson

README.md

@@ -168,8 +168,8 @@ If no Julia graphics backend is available when PyPlot is imported, then
 
 ### Choosing a Python GUI toolkit
 
-Only the [wxWidgets](http://www.wxwidgets.org/),
-[GTK+](http://www.gtk.org/) (version 2 or 3), and [Qt](http://qt-project.org/) (via the
+Only the [Tk](http://www.tcl.tk/), [wxWidgets](http://www.wxwidgets.org/),
+[GTK+](http://www.gtk.org/) (version 2 or 3), and [Qt](http://qt-project.org/) (version 4 or 5; via the PyQt5, 
 [PyQt4](http://wiki.python.org/moin/PyQt4) or
 [PySide](http://qt-project.org/wiki/PySide)), Python GUI backends are
 supported by PyPlot.  (Obviously, you must have installed one of these
@@ -183,7 +183,7 @@ using PyCall
 pygui(gui)
 using PyPlot
 ```
-where `gui` can currently be one of `:wx`, `:gtk`, or `:qt`.  You can
+where `gui` can currently be one of `:tk`, `:gtk3`, `:gtk`, `:qt5`, `:qt4`, `:qt`, or `:wx`. You can
 also set a default via the Matplotlib `rcParams['backend']` parameter in your
 [matplotlibrc](http://matplotlib.org/users/customizing.html) file.