Fix boxplot conversion by mapping 'none' colors to transparent rgba

[robertoffmoura]

mpl_to_plotly crashes when converting boxplots:

ValueError: Invalid value of type 'builtins.str' received for the 'color' property of scatter.marker
    Received value: 'none'

Boxplot outlier markers use facecolor="none" (fully transparent) in matplotlib. The converter passes that color verbatim into the plotly trace, and plotly.py's validators reject "none" for color property.

Fix: transparent matplotlib colors are mapped to rgba(0,0,0,0), which plotly accepts.

Snippet to reproduce:

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import numpy as np
import plotly.tools as tls

fig, ax = plt.subplots()
ax.boxplot(np.random.randn(100, 4))
fig.savefig("boxplot_mpl.png")

p = tls.mpl_to_plotly(fig)   # raised ValueError before the fix
p.write_image("boxplot_plotly.png")