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/shared_library/Rsconscript

@@ -1,13 +1,11 @@
-default do
-  Environment.new do |env|
-    env["CPPPATH"] << "src/lib"
-    libmine = env.SharedLibrary("mine", glob("src/lib/*.c"))
-    env.Program("test-shared.exe",
-                glob("src/*.c"),
-                "LIBPATH" => %w[.],
-                "LIBS" => %w[mine])
-    env.build_after("test-shared.exe", libmine)
-    env.Program("test-static.exe",
-                glob("src/**/*.c"))
-  end
+env do |env|
+  env["CPPPATH"] << "src/lib"
+  libmine = env.SharedLibrary("mine", glob("src/lib/*.c"))
+  env.Program("test-shared.exe",
+              glob("src/*.c"),
+              "LIBPATH" => %w[.],
+              "LIBS" => %w[mine])
+  env.build_after("test-shared.exe", libmine)
+  env.Program("test-static.exe",
+              glob("src/**/*.c"))
 end