Add target 'install' to Makefile

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add help function to summarize contributions

* src/git2r_revwalk.c/h (git2r_revwalk_contributions): Help function
  to summarize to contributions directly in C code. This gives a
  improved performance compared to first fetch all commits and then
  summarize.

* src/git2r.c: Update callMethods

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Improved performance when summarizing contributions

* R/contributions.r (contributions): Use C
  function (git2r_revwalk_contributions) to summarize
  contributions. Add breaks 'year' and 'quarter'.

* man/contributions-methods.Rd: Update documentation

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add 'year' and 'quarter' to breaks argument in plot method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update NEWS

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add script to build src/git2r.c

* scripts/build_git2r_c.r: Initial work to automatically build
  callMethods in src/git2r.c from .Call methods.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>