Add fields URL and BugReports to DESCRIPTION
Add check for LibSSH2 in configure.ac

* configure.ac: Add check for ssh2 and add substitution variable
  GIT2R_HAVE_SSH2. Changed check for ssl to build without support for
  HTTPS if OpenSSL not found. Add substitution variable
  GIT2R_HAVE_SSL.

* configure: Rebuild
Add GIT2R_HAVE_SSL and GIT2R_HAVE_SSH2 to Makevars.in