Added more tests

* tests/repository.R: Added tests to initialize a repository and check
  the state of the repository.
Minor update of tests

* tests/repository.R: Changed to use tempfile method to generate a
  path to test repository.
Refactoring of the S4 class git_tag
Updated author list
Merge branch 'master' of https://github.com/ropensci/git2r

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Added summary method to S4 class git_tag

R/tag.r: Added summary method and updated roxygen documentation
Refactoring of function to list tags

* src/git2r.c (tags): Refactored function to list tags

* src/git2r.c (init_tag): New function to initialize a S4 class
  git_tag object
Minor fix to roxygen documention

* R/tag.r (summary): Fixed examples tag
Added target in Makefile to build doc
Rebuilt documentation
Added S4 class git_tree

* R/tree.r: Added S4 class git_tree.
Initial work with checkout method

* R/checkout.r (checkout): Added a generic method checkout and a
  method for git_repository. The method calls the C code method
  checkout.

* src/git2r.c (checkout): Added checkout function. However, it
  currently performs nothing.
Updated documentation
Updated NAMESPACE and DESCRIPTION
Fixed typo in configure.ac