Added check that repository path is not NA

* src/git2r.c (get_repository): Added check that S4 class
  git_repository path slot is not NA

* src/git2r.c (is_repository): Added similar checks as get_repository
  to path argument before usage of path to open repository.
Moved git2r repository C code to separate file

* Refactoring to split git2r.c into several smaller files

* src/git2r_repository.h: Updated

* src/git2r_repository.c: Added