Add more ImageDraw tests

Most of the `ImageDraw` functions are now tested.

Found some problems and questions (noted in the test file with `FIXME`):

* [x] The [docs](http://pillow.readthedocs.org/en/latest/reference/ImageDraw.html) say `arc`, `chord` and `pieslice` take a bounding box parameter of either `[(x0, y0), (x1, y1)]` or `[x0, y0, x1, y1]` but only the latter works.
* [x] `arc`: "Draws an arc (a portion of a circle outline) between the start and end angles". But I was caught out because the angle begins at 3 o'clock and not 12 o'clock. Is this standard? Should it be documented?
* [x] `arc` mixes up a paremter name in docs and code. [Docs](http://pillow.readthedocs.org/en/latest/reference/ImageDraw.html#PIL.ImageDraw.PIL.ImageDraw.Draw.arc) say "`PIL.ImageDraw.Draw.arc(`*xy, start, end, fill=None*)" but give the parameter as "**outline** – Color to use for the outline." The code uses parameter `fill`. The function draws an arc, which is just a line with no fill, so `outline` makes more sense. How should this be best dealt with without breaking code which already uses this?