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/two_sources/Rsconscript

@@ -1,6 +1,4 @@
-default do
-  Environment.new(echo: :command) do |env|
-    env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
-    env.Program('two_sources.exe', ['one.o', 'two.c'])
-  end
+env(echo: :command) do |env|
+  env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
+  env.Program('two_sources.exe', ['one.o', 'two.c'])
 end