Add reset functionality

* R/reset.r: Add method to reset (soft, mixed or hard) to a commit.

* src/git2r_reset.c/h: Add function git2r_reset

* Add documentation, update NAMESPACE and add test cases

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add 'Not implemented' error message to pull method

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