Add test case for is.detached method

* tests/checkout.R: Add test case for check if HEAD of repository is
  detached using method is.detached
Added method to check if HEAD of repository is detached

* R/repository.r (is.detached): Added generic is.detached and method
  for S4 class git_repository to check if HEAD of repository is detached.

* man/is.detached-methods.Rd: Added documentation

* src/git2r_repository.c/h (is_detached): Added C code to check if
  HEAD of repository is detached.

* src/git2r.c: Added "is_detached" to callMethods

* NAMESPACE: Updated