Implement support for global getters

[stalniy]

As an developer I want to use global getters So that, I can write cleaner code

def('username', 'admin');

it('defines getter on global scope', function() {
  expect(username).to.equal('admin');
});

[stalniy]

in order to avoid naming collisions it's better add some prefix to each defined global variable, something like $. So,

subject(function() {
  return new Object()
});

it('is accessible via $subject', function() {
  expect($subject).to.be.an('object');
});