Add shortcut method for creating environments - close #149

Add env() method to create environments.
Process all environments created at build script top level before
executing any autoconf-enabled tasks, or if no tasks are specified by
the user.

build_tests/tasks/tasks.rb

@@ -2,7 +2,7 @@ task "one" do
   puts "one"
 end
 
-task "two", deps: ["one", "three"] do
+task "two", depends: ["one", "three"] do
   puts "two"
 end