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
Added progress argument to clone method

* R/repository (clone): Added argument progress = TRUE to clone method
Added clone progress indicator

* src/git2r.c (clone): Added use of clone and checkout options.

* src/git2r.c (clone_progress): Added callback function to be called
  during the clone process to give progress feedback.
Updated documentation
Updated DESCRIPTION
Merge pull request #20 from ropensci/add-clone-text-progressbar

Add clone progress
Added more tests

* tests/collaboration.R: Test collaboration between a bare repository
  and two clones. Not all functionality in package yet to test pull
  and push.
Minor roxygen fixes and rebuilt documentation
Fixed contributions method

* R/contributions.r: Fixed error when calculating contributions by
  user. Changed to use both author, email and when to create an index.
Added method when to facilitate extraction of time

* R/when.r (when): Added generic method when to facilitate extraction
  of the time slot as a character vector from the S4 classes
  git_commit, git_signature, git_tag and git_time.

* tests/when.R: Test cases to test method when

* man/when-methods.Rd: Added

* Updated NAMESPACE and DESCRIPTION
Minor fix documentation
Minor refactoring of S4 class git_commit show method
Minor refactoring of S4 class git_signature show method
Initial work with config file management

* R/config.r (config): Added file with config method to set user.name
  and user.email locally in a repository.

* man/config-methods.Rd: Added file

* Updated NAMESPACE and DESCRIPTION
Minor update of documentation
Changed ' to " in code and roxygen documentation
Updated DESCRIPTION
Changed ' to " in R code and roxygen documentation