Makefile to automate common development tasks

[aoles]

Have the following targets similar to https://github.com/richfitz/storr/blob/master/Makefile

• [x] R CMD INSTALL
• [x] R CMD build
• [x] R CMD check
• [ ] build vignette(s)
• [ ] test
• [x] roxygenize
• [x] generate README.md
• [x] rebuild docs/ with pkgdown
• [x] clean docs/
• [x] commit docs/ to gh-pages branch
• [ ] spell checking of man pages