Code review of git2r_remote_* C functions

* Move git2r_remote_* C functions from src/remotes.c/h to
  src/git2r_remote.c/h

* src/remotes.c/h: Deleted

* src/Makevars.in/win: Updated to deleted git2r_remotes.c

* src/git2r.c: Remove include git2r_remotes.h

* src/git2r_remote.c: Fix missing doxygen documentation. Consistent
  argument checking
Add error message for string vector argument check
Update NEWS
Update documentation
Cleanup after remote tests
Fix memory leak in remote_remove

* src/git2r_remote.c (git2r_remote_remove): Free allocated git_remote
  objects.