Fix conversion of matplotlib polar plots

[robertoffmoura]

mpl_to_plotly currently crashes for polar plots:

KeyError: 'bottom'

Fix:

• polar axes are detected and converted to a plotly layout.polar subplot instead of cartesian axes
• angular and radial tick values and labels are exported
• polar background, grid, and frame colors are taken from matplotlib; the exporter runs tuple/RGBA colors through export_color, so custom frame and grid colors are picked up

Snippet to reproduce:

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

theta = np.linspace(0, 2 * np.pi, 400)

fig, ax = plt.subplots(subplot_kw={"projection": "polar"})
ax.plot(theta, 1 + 0.5 * np.sin(3 * theta), color="crimson", linewidth=2)
fig.savefig("polar_mpl.png")

p = tls.mpl_to_plotly(fig)   # raised KeyError: 'bottom' before
p.write_image("polar_plotly.png")

matplotlib	plotly before	plotly after
	KeyError: 'bottom'