Initial work with clone method

* R/repository.r (clone): Added method clone. It currently fails to
  clone https://github.com/ropensci/git2r.git since git2r package is
  not yet built with SSL support. However it works fine to clone a
  local bare repository.

* src/git2r.c (clone): Added function clone. No support yet for
  progressbar.

* NAMESPACE: Added export(clone)

* man/clone.Rd: Added
Fixed SSL build support

* configure.ac: Added check for libraries ssl and crypto

* src/Makevars.in: Added define -DGIT_SSL
Merge pull request #17 from ropensci/clone-functionality

Clone functionality