Simplify set_boxstyle Accepts section of FancyBboxPatch

Change it to something that can actually be passed by docscrape, correctly included in the and displayed in the setp docs.

Before setp would return something like: 

```
boxstyle: ==========   ==============   ==========================   Class        Name             Attrs                        ==========   ==============   ==========================   Circle       ``circle``       pad=0.3                      DArrow       ``darrow``       pad=0.3                      LArrow       ``larrow``       pad=0.3                      RArrow       ``rarrow``       pad=0.3                      Round        ``round``        pad=0.3,rounding_size=None   Round4       ``round4``       pad=0.3,rounding_size=None   Roundtooth   ``roundtooth``   pad=0.3,tooth_size=None      Sawtooth     ``sawtooth``     pad=0.3,tooth_size=None      Square       ``square``       pad=0.3                      ==========   ==============   ==========================
```

i.e. a list rendered as one line.

now it returns:
```
boxstyle: [u'square', u'sawtooth', u'roundtooth', u'darrow', u'round4', u'rarrow', u'larrow', u'circle', u'round']
```

The html docs used to render as 

http://matplotlib.org/api/patches_api.html (see set_boxstyle section)

now 

renders as 

http://jenshnielsen.github.io/mplddocs/api/patches_api.html

Edit: Correct link
This should be `[' | '.join(str(i) for i in _styles.keys())` to match the style for ACCEPTS.