Added independent chart examples.

This PR fixes #1014.
I think we can do even better. Why can't we just pas the data frame directly to `BoxPlot`? I think users should not have to do any of this `astype` and dict business. I think this examples should read:
```
import pandas as pd

# we throw the data into a pandas df
from bokeh.sampledata.olympics2014 import data
df = pd.io.json.json_normalize(data['data'])

# we filter by countries with at least one medal and sort
df = df[df['medals.total'] > 0]
df = df.sort("medals.total", ascending=False)

# and finally we drop the dict into our BoxPlot chart
from bokeh.charts import BoxPlot
boxplot = BoxPlot(
    df, fields=("medals.gold", "medals.silver", "medal.bronze"), 
    marker='circle', outliers=True, title="boxplot test",
    xlabel="medal type", ylabel="medal count", 
    width=600, height=400, filename="boxplot.html"
)

boxplot.show()
```