gghinton 0.1.0

• Initial release.
• geom_hinton() and stat_hinton() for drawing Hinton diagrams in ggplot2.
• scale_fill_hinton() for the conventional signed-data colour scheme.
• theme_hinton() for a clean, grid-free panel theme.
• matrix_to_hinton() and as_hinton_df() for converting matrices to tidy data frames.