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.

Let's use [`AbstractBuild#getEnvironment(TaskListener)`](http://javadoc.jenkins-ci.org/hudson/model/AbstractBuild.html#getEnvironment(hudson.model.TaskListener)), as `getEnvironment()` is deprecated.