Access all environment variables using a simple hash map.

This is a modified version of pull request #13, which I closed after seeing the error in putting in the EnvInject plugin dependency. Here is what my version will accomplish:

-Sets a hash map named "environment" that contains all of the environmental variables. This makes the process of getting variables more simple. (No EnvInject dependency)

-Added into the help file how to use the environment hash map.

Why don't you also catch IOException?
Let's use `printStacktrace(PrintWriter)` with `listener.getLogger()`. It allows users to see the cause on a build log.
And it might be better to set empty `EnvVars` to `envVars` to avoid NPE.