Fix memory leak

* src/git2r.c (default_signature): Added missing git_signature_free
Updated documentation

* src/git2r.c: Ongoing work to improve doxygen documentation of
  functions.
Fixed error checking when fetching remote url

* src/git2r.c (remote_url): Now checks for error after call to
  git_remote_load
Removed unused variable

* src/git2r.c (revisions): Removed unused variable oid_hex
Splitted large function

* src/git2r.c (revisions): Splitted function revisions, moved
  initialization of S4 class git_commit to new function init_commit.

* src/git2r.c: Fixed compiler warnings discarding const qualifier from
  pointer target type.
Fixed duplicated variable name

* src/git2r.c (remote_url): Corrected introduced misstake when fixing
  error checking. The inner variable remote was renamed to same name
  as function argument remote. The inner variable now renamed to r.
Added slot hex to S4 class git_commit

* R/commit.r: The new hex slot contains the 40 char hexadecimal string.

* man/git_commit-class.Rd:  Updated documentation.
Updated S4 class git_commit show method

* R/commit.r (show): Output now includes the commit hex SHA
Merge branch 'master' of https://github.com/ropensci/git2r

Merged README.md