Add support for environment data

By default, protos reads environment data from a JSON file with the
same name of the current environment, in the application's root
directory.

For example, when running on the development environment, if there is
a JSON file called development.json, it will be read and its data
will be used as the environment data.

Environment data can be retrieved using the protos.env() method. Assuming
there is an environment JSON file containing the following:

{
  name: "ernie"
  age: 30
}

Individual environment data keys can be retrieved, as follows:

protos.env('name'); // returns "ernie"
protos.env('age');  // returns 30
protos.env()            // returns {name: 'ernie', age: 30}

Environment data has many useful applications, such as setting individual
application behavior based on the environment, as well as setting specific
environment configuration for databases and storages.

To override the environment data file for each environment, you can add an
envData entry to the boot.js file, as such:

envData: {
  development: "testing.json",
  debug: "testing.json",
  production: "deploy.json"
}

This allows fine grained control for environment data files and locations.