add fallback for require that using aliases

[lucaoliano]

I added a fallback in module resolution, giving the possibility to add an alias for a module.
In the karma.conf.js, aliases can be specified like below:

module.exports = function (config) {
  config.set({
    ...
    commonjsPreprocessor: {
      ...
      alias: {
        'module_a': '../node_modules/common_module_a/lib/common_module.js',
        'module_b': '../node_modules/common_module_b/lib/common_module.js'
      }
    }
    ...
  });
};

In this way require('module_a') is resolved to '../node_modules/common_module_a/lib/common_module.js'

This resolves the issue #52

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[lucaoliano]

I signed it!

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