Series.hist() fails for String Series
Not sure if that's a pandas or a mathplotlib bug.

``` python
import pandas
df = pandas.DataFrame({"year":[1,2,3,4],"name": [u"abc", u"bcd", u"cde", u"abc"]})
df["year"].hist() # Works
df["name"].hist()
TypeError                                 Traceback (most recent call last)
<ipython-input-11-79a5ab7bbbd5> in <module>()
----> 1 df["name"].hist()

C:\portabel\Python27\lib\site-packages\pandas\tools\plotting.pyc in hist_series(self, by, ax, grid, xlabelsize, xrot, ylabelsize, yrot, figsize, **kwds)
   2113         values = self.dropna().values
   2114 
-> 2115         ax.hist(values, **kwds)
   2116         ax.grid(grid)
   2117         axes = np.array([ax])

C:\portabel\Python27\lib\site-packages\matplotlib\axes.pyc in hist(self, x, bins, range, normed, weights, cumulative, bottom, histtype, align, orientation, rwidth, log, color, label, stacked, **kwargs)
   8325             # this will automatically overwrite bins,
   8326             # so that each histogram uses the same bins
-> 8327             m, bins = np.histogram(x[i], bins, weights=w[i], **hist_kwargs)
   8328             m = m.astype(float) # causes problems later if it's an int
   8329             if normed and not stacked:

C:\portabel\Python27\lib\site-packages\numpy\lib\function_base.pyc in histogram(a, bins, range, normed, weights, density)
    174             else:
    175                 range = (a.min(), a.max())
--> 176         mn, mx = [mi+0.0 for mi in range]
    177         if mn == mx:
    178             mn -= 0.5

TypeError: coercing to Unicode: need string or buffer, float found
```
