Update documentation of specgram

Mention that plotted data is in dB
check `ret is False` in Timer._on_timer

The docstring states:

> Functions can return False if they should not be called any more.

Cleanup commit 2f11dee changed this logic,
so any return value whose boolean interpretation is False (e.g. None)
would be unregistered.

fixes #1492
DOC: Update return value units in specgram
add comment to protect against future 'cleanup' of `if ret is False`
use ret == False instead of ret is False

per review
set_cmap should not require an active image

Before this commit, it was not possible to change the default colormap if there
wasn't an active colorable artist, i.e. if plt.gci() returned None.

There's no reason why we should force this, though, since part of what
plt.set_cmap does is setting the default colormap for all FUTURE colorable
artists which will be created.

Colormaps which are currently exposed as their own functions in pyplot, such as
plt.gray(), plt.hsv(), plt.hot() and plt.jet() make no such restriction, they
do not complain if plt.gci() happens to return None.

Prior to this commit
```
In [2]: plt.set_cmap('bwr')
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-2-fa42f8679abe> in <module>()
----> 1 plt.set_cmap('bwr')

/home/pi/.local/lib/python2.7/site-packages/matplotlib/pyplot.pyc in set_cmap(cmap)
   2029         im.set_cmap(cmap)
   2030     else:
-> 2031         raise RuntimeError('You must first define an image, eg with imshow')
   2032
   2033     draw_if_interactive()

RuntimeError: You must first define an image, eg with imshow

In [3]:
```

After this commit, no error is thrown, and the appropriate colormap is activated.
```
In [2]: plt.set_cmap('bwr')

In [3]:
```
DOC: github ribbon does not cover up index link

thanks @jenshnielsen for spotting the fact that the previous "solution"
didn't actually work as intended.
Make README.txt consistent reStructuredText

It seems that the README file is formatted as reStructuredText, but not
consistently.
pgf: documentation enhancements
Merge pull request #1510 from pwuertz/pgf-doc-enhancements

pgf: documentation enhancements
pgf: documentation enhancements
Merge pull request #1525 from pwuertz/pgf-doc-enhancements

pgf: documentation enhancements
Merge pull request #1524 from martin-ueding/readme

Make README.txt consistent reStructuredText
fix 2 html color names

There are two garbled up lines resulting in one missing colorname and one colorname assigned to the wrong html color code.
Merge pull request #1527 from megies/patch-1

fix color names for paleturquoise and palevioletred
fix 2 html color names

There are two garbled up lines resulting in one missing colorname and one colorname assigned to the wrong html color code.
Merge pull request #1527 from megies/patch-1

fix color names for paleturquoise and palevioletred
Merge pull request #1489 from gitj/master

Documentation update for specgram
Merge pull request #1515 from ivanov/set-cmap

set_cmap should not require an active image
Merge pull request #1523 from ivanov/gh-ribbon-fixed

DOC: github ribbon does not cover up index link