BUG: plot(kind='bar') ignores `use_index` keyword
with `df = pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])`.

From `df.plot(kind='bar', use_index=False)`:

![bar_false](https://f.cloud.github.com/assets/1312546/2053712/c892ee7c-8aab-11e3-9ab4-2810cf9adbd5.png)

From `df.plot(use_index=False)`:

![line_false](https://f.cloud.github.com/assets/1312546/2053715/d6ad8328-8aab-11e3-8495-604fdaffe9f2.png)

Most of the time it doesn't make sense to do a bar plot and ignore the groups formed by the index, but I had an index with a bunch of long strings and just wanted to see the distribution (pretty much a histogram), so I didn't care about the index.

PR inbound.
