Fix bugs related to bottom kwarg in step histograms

Conflicts:

	lib/matplotlib/axes.py
	lib/matplotlib/tests/baseline_images/test_axes/hist_step.png
Merge pull request #2409 from neggert/hist_bottom_fix2

Fix bugs related to bottom kwarg in step histograms
improve docstring and add test fot to_rgb(<float>)

Even if the docstring was saing it, emphasis that to_rgb/to_rgba do not
accept float as input, but string representation of a float.

As one might be tempted to had float->gray conversion add failing test
in case the functionality is added.

Side fixes :
    change tuple([value]*3) to (value,)*3 for speed

In [18]: %timeit  tuple([0.4]*3)
1000000 loops, best of 3: 505 ns per loop

In [19]: %timeit  (0.4,)*3
10000000 loops, best of 3: 23.5 ns per loop

In [20]:  (0.4,)*3 == tuple([0.4]*3)
Out[20]: True

Closes gh-2609
BLD Fix build failure on Python 3.4b1 for Windows Windows

As of version 3.4b1 the official CPython distribution for Windows uses Tcl/Tk 8.6.
Merge pull request #2613 from cgohlke/patch-1

BLD Fix build failure on Python 3.4b1 for Windows
Merge remote-tracking branch 'upstream/v1.3.x'

Conflicts:
	lib/matplotlib/axes/_axes.py
minor pep8 to fix failing master builds.
Merge pull request #2626 from tacaswell/minor_pep8

minor pep8 to fix failing master builds.
Merge pull request #2610 from Carreau/close-2609

improve docstring and add test fot to_rgb(<float>)
Added new-style formatter function.

Added class `StrMethodFormatter` which is identical to `FormatStrFormatter`
only for new-style ('{}'.format) formatting methods.