STY: remove unused imports in plot_directive
STY: pep8 2 lines between functions

pep8 for plot directive.
MNT: remove Python 2.7 copy of relpath

Matplotlib now depends on Python 2.6, which has relpath
STY: typos in plot_directive docstrings

Well, behaviour -> behavior is against my origins, but hey, we're all
American now.
BUG: don't show parens, comma when no source link

plot_directive displaying empty parens when no figures and no source
required.  Was also prepending a comma to list of images when there was
source links not required.
Merge pull request #3257 from matthew-brett/refactor-plot-directive

MRG: refactor and bugfixes for plot_directive