First CRAN release

• Better mandelbrot_palette
• Implement transforms for plot.mandelbrot
• CRAN-related changes

Mandelbrot set generation

• Main function mandelbrot (and alt interface mandelbrot0) generates Mandelbrot set
• Helper functions and generics for mandelbrot objects:
  • as.data.frame.mandelbrot
  • plot.mandelbrot
• Support for generating good palettes for visualisation (mandelbrot_palette)