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)
actually now that I think about it, I'm not sure your example will work.

I'm under the assumption that passing the `--password` flag would pass back `{:password => ''}`

If this is correct than just testing for the key won't work, because it will always have the key.

```ruby
if self.options[:token]
  if self.options[:token].length > 0
    opts[:access_token] = self.options[:token]
  else
    print 'Token: '
    token = STDIN.noecho(&:gets).chomp
    opts[:access_token] = token if token.length > 0
  end
elsif self.options[:password]
  if self.options[:password].length > 0
    opts[:password] = self.options[:password]
  else
    print 'Password: '
    token = STDIN.noecho(&:gets).chomp
    opts[:password] = password if password.length > 0
  end
else
  raise Thor::RequiredArgumentMissingError.new("No value provided for option --password or --token")
end
```