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/direct/c_program.rb

@@ -1,5 +1,3 @@
-default do
-  Environment.new do |env|
-    env.Program("test.exe", glob("*.c"), direct: true)
-  end
+env do |env|
+  env.Program("test.exe", glob("*.c"), direct: true)
 end

build_tests/direct/c_shared_library.rb

@@ -1,7 +1,5 @@
-default do
-  Environment.new do |env|
-    lib = env.SharedLibrary("mylib", ["two.c", "three.c"], direct: true)
-    program = env.Program("test.exe", "main.c", "LIBS" => ["mylib"], "LIBPATH" => ["."])
-    env.depends(program, lib)
-  end
+env do |env|
+  lib = env.SharedLibrary("mylib", ["two.c", "three.c"], direct: true)
+  program = env.Program("test.exe", "main.c", "LIBS" => ["mylib"], "LIBPATH" => ["."])
+  env.depends(program, lib)
 end