Add an excel renderer to the package

[robbertjan94]

This PR fixes #77: implementing an excel renderer.

The excel renderer is based on the xlsxwriter package.

Example:

import pandas as pd
from sklearn import datasets
import statsmodels.api as sm
from stargazer.stargazer import Stargazer

diabetes = datasets.load_diabetes()
df = pd.DataFrame(diabetes.data)
df.columns = ['Age', 'Sex', 'BMI', 'ABP', 'S1', 'S2', 'S3', 'S4', 'S5', 'S6']
df['target'] = diabetes.target

est = sm.OLS(endog=df['target'], exog=sm.add_constant(df[df.columns[0:4]])).fit()
est2 = sm.OLS(endog=df['target'], exog=sm.add_constant(df[df.columns[0:6]])).fit()
stargazer = Stargazer([est, est2])

Calling the excel renderer works similar to the html and LaTeX renderers:

stargazer.render_excel(filename='test.xlsx')

Which returns the following table in the test.xlsx file: