BUG: barplot with log=True not working for values smaller than 1
```
s = pd.Series([0.1, 0.01, 0.001], index=['a', 'b', 'c'])
s.plot(kind='bar', log=True)
```

gives:

```
TypeError                                 Traceback (most recent call last)
<ipython-input-13-75b989d29042> in <module>()
      1 s = pd.Series([0.1, 0.01, 0.001], index=['a', 'b', 'c'])
----> 2 s.plot(kind='bar', log=True)

C:\Anaconda\lib\site-packages\pandas\tools\plotting.pyc in plot_series(data, kind, ax, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, label, secondary_y, **kwds)
   2517                  yerr=yerr, xerr=xerr,
   2518                  label=label, secondary_y=secondary_y,
-> 2519                  **kwds)
   2520 
   2521 

C:\Anaconda\lib\site-packages\pandas\tools\plotting.pyc in _plot(data, x, y, subplots, ax, kind, **kwds)
   2323         plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
   2324 
-> 2325     plot_obj.generate()
   2326     plot_obj.draw()
   2327     return plot_obj.result

C:\Anaconda\lib\site-packages\pandas\tools\plotting.pyc in generate(self)
    921         self._compute_plot_data()
    922         self._setup_subplots()
--> 923         self._make_plot()
    924         self._add_table()
    925         self._make_legend()

C:\Anaconda\lib\site-packages\pandas\tools\plotting.pyc in _make_plot(self)
   1884                 w = self.bar_width / K
   1885                 rect = bar_f(ax, self.ax_pos + (i + 0.5) * w, y, w,
-> 1886                              start=start, label=label, **kwds)
   1887             self._add_legend_handle(rect, label, index=i)
   1888 

C:\Anaconda\lib\site-packages\pandas\tools\plotting.pyc in f(ax, x, y, w, start, **kwds)
   1823         if self.kind == 'bar':
   1824             def f(ax, x, y, w, start=None, **kwds):
-> 1825                 start = start + self.bottom
   1826                 return ax.bar(x, y, w, bottom=start,log=self.log, **kwds)
   1827         elif self.kind == 'barh':

TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
```

This was working in 0.14.1
