Support password/token prompt

Closes #60

Looking for feedback, and advice on the best way to test manually entering a prompt.

I had a lot of build issues trying to use [ruby-password](http://www.caliban.org/ruby/ruby-password.shtml)

So I extracted out what was actually needed, and added it to the gem.

You can find a GitHub mirror of the source code [here](https://github.com/savonix/ruby-password/blob/master/lib/password.rb)
So it turns out that if you set the flag by itself, this is what happens:

```ruby
From: /Users/mark/.gem/ruby/2.1.2/gems/teachers_pet-0.3.3/lib/teachers_pet/actions/base.rb @ line 25 TeachersPet::Actions::Base#octokit_config:

    20:           login: self.options[:username],
    21:           # Organizations can get big, pull in all pages
    22:           auto_paginate: true
    23:         }
    24:
 => 25:         binding.pry
    26:         if self.options[:token]
    27:           if self.options[:token].length > 0
    28:             opts[:access_token] = self.options[:token]
    29:           else
    30:             print 'Please enter your GitHub token: '

>> self
=> #<TeachersPet::Actions::CreateRepos:0x007febf3345f30
 @options=
  {:public=>false,
   :students=>"./students",
   :username=>"tarebyte",
   :api=>"https://api.github.com/",
   :web=>"https://www.github.com/",
   :organization=>"tarebyte-test-org",
   :repository=>"hello-there",
   :password=>"password"},
 @organization="tarebyte-test-org",
 @public_repos=false,
 @repository="hello-there",
 @students={"tarebyte"=>["tarebyte"]}>
```

:confused: