ENH: Scatter plot now supports errorbar

This is enhancement for #5638. I sometimes want to plot scatter with errorbars, thus I've refactored to support it.

![figure_1](https://cloud.githubusercontent.com/assets/1696302/2632517/21edcfd6-be65-11e3-9cd9-97bad65c8129.png)

Also, this includes following fixes. I've also modified tests to check the number of errorbars actually drawn.

- There are some types which expected to work, but results in `ValueError`.
  - `float` (MPL can accept it)
  - `unicode` (in Python 2.x)
  - Other list-like, such as `iterator`

- A bug related to time series.
```
>>> s = Series(rand(6), index=range(6), name='x')
>>> err_df = DataFrame(rand(6, 3) / 10, index=range(6), columns=['x', 'y', 'z'])
>>> s.plot(yerr=err_df)               # This works, errorbar appears using err_df x column

>>> s.index = tm.makeDateIndex(k=6)
>>> s.plot(yerr=err_df)               # But this doesn't, errorbar doesn't appear
```