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/simple/register_target_in_build_hook.rb

@@ -1,10 +1,8 @@
-default do
-  Environment.new do |env|
-    env.Program("simple.exe", Dir["*.c"])
-    env.add_build_hook do |builder|
-      if builder.target.end_with?(".o")
-        env.Disassemble("#{builder.target}.txt", builder.target)
-      end
+env do |env|
+  env.Program("simple.exe", Dir["*.c"])
+  env.add_build_hook do |builder|
+    if builder.target.end_with?(".o")
+      env.Disassemble("#{builder.target}.txt", builder.target)
     end
   end
 end