scatter plot and hexbin plot lose x-axis when colorbar is included.
closes https://github.com/pandas-dev/pandas/issues/10611 
closes https://github.com/pandas-dev/pandas/issues/10678. 

The x-axis for scatter plot and hexbin plot disappears when colorbar is included. This seems to be because colorbar axis is looped through in `_handle_shared_axes`: https://github.com/pandas-dev/pandas/blob/8a5830305d2d9a087fbe46bd968218104ffdfc49/pandas/plotting/_core.py#L426
after discussing with @TomAugspurger (issue https://github.com/pandas-dev/pandas/issues/10611), we decided to try adding the attribute `__is_pandas_colorbar` to colorbar axis object and skipping it during handling of shared axes. 

I've done some tests that seem to fix the issue. But we may need more tests:
```
%matplotlib inline
import matplotlib.pylab as pl
from mypandas import pandas as pd
import numpy as np
random_array = np.random.random((1000,3))

df = pd.DataFrame(random_array,columns=['A label','B label','C label'])
df.plot.scatter('A label','B label',c='C label', PATCH_MODE_FLAG = False);pl.title('pandas current version');
df.plot.scatter('A label','B label',c='C label', PATCH_MODE_FLAG = True);pl.title('patch fixing x-axis');

df.plot.hexbin('A label','B label', gridsize=25, PATCH_MODE_FLAG = False);pl.title('pandas current version');
df.plot.hexbin('A label','B label', gridsize=25, PATCH_MODE_FLAG = True);pl.title('patch fixing x-axis');
```

![image](https://user-images.githubusercontent.com/26352146/37747996-49642266-2d58-11e8-8c6f-c382267e79db.png)
![image](https://user-images.githubusercontent.com/26352146/37747999-4d88d986-2d58-11e8-8445-f1348c35d892.png)
![image](https://user-images.githubusercontent.com/26352146/37748001-5109d93e-2d58-11e8-9f6f-73a68329f731.png)
![image](https://user-images.githubusercontent.com/26352146/37748005-54beb590-2d58-11e8-8233-4e26b135fdad.png)
