Use personal access tokens when cloning repos

I ran into issues where I could not pull down our private creds repo (docs-pcfp-site-credentials) when I ran <code>bundle exec bookbinder push_local_to_staging</code>. With this change we send the GITHUB_API_TOKEN (personal access key) in the url according to github docs. The clone URI is now assumed to be https instead of git. I'm not sure this is the "right" way to fix this issue, but worked for my needs. It feels weird to change git to https, but there are probably other reasons why that might be good (firewalls), so I didn't sweat it too much. Please advise if I should go about this a different way and I'll take another shot. 

Here's a before and after well as a test run: 

    ➜  docs-book-pcfp git:(master) ✗ bundle exec bookbinder push_local_to_staging                                                                                                                
    Processing pivotal-cf/docs-pcfp-site-credentials
    git clone '--' 'git@github.com:pivotal-cf/docs-pcfp-site-credentials' '/var/folders/44/kxt0zwwx12bb7vkcznnd7_8m0000gp/T/d20141124-8557-1ms722l/docs-pcfp-site-credentials'  2>&1:Cloning into '/var/folders/44/kxt0zwwx12bb7vkcznnd7_8m0000gp/T/d20141124-8557-1ms722l/docs-pcfp-site-credentials'...
    Permission denied (publickey).
    fatal: Could not read from remote repository.

    Please make sure you have the correct access rights and the repository exists.
    ➜  docs-book-pcfp git:(master) ✗ (cd ../bookbinder && git checkout api_tokens )  
    Switched to branch 'api_tokens'

    ➜  docs-book-pcfp git:(master) ✗ bundle exec bookbinder push_local_to_staging  
    Processing pivotal-cf/docs-pcfp-site-credentials
    API endpoint: https://api.run.pivotal.io
    Authenticating...
    OK
    Targeted org pcfp
    ....
    This URL will expire in 2 hours, so if you need to share it, make sure to save a copy now.

    #This is after merging to master, contains changed code
    ➜  bookbinder git:(master) rake
    /Users/jkruck/.rvm/rubies/ruby-2.1.1/bin/ruby -I/Users/jkruck/.rvm/gems/ruby-2.1.1/gems/rspec-core-3.1.7/lib:/Users/jkruck/.rvm/gems/ruby-2.1.1/gems/rspec-support-3.1.2/lib /Users/jkruck/.rvm/gems/ruby-2.1.1/gems/rspec-core-3.1.7/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb
    Run options: exclude {:integration=>true}
    .................................................*..................................................................................................................................................................................................................................................................*.........................*....*.............................

    Pending:
      Bookbinder::MiddlemanRunner behaves like a ShellOut
        # Not yet implemented
        # ./spec/middleman_runner_spec.rb:23
      Sieve#links_from when the page is found and it does not have a valid html body
        # No reason given
        # ./spec/sieve_spec.rb:32
      PdfGenerator generates a PDF from the specified pages and header
        # No reason given
        # ./spec/pdf_generator_spec.rb:20
      PdfGenerator when generating pages from a live web-server generates a PDF from a live web-page and header
        # No reason given
        # ./spec/pdf_generator_spec.rb:34

    Finished in 44.2 seconds (files took 1.95 seconds to load)
    369 examples, 0 failures, 4 pending

    Randomized with seed 44177
    
Thanks, 
Josh