add features to Nginx::Time

 * Nginx::Time.time (renamed from now)
 * Nginx::Time.http_time
 * Nginx::Time.cookie_time
 * Nginx::Time.utc_time
 * Nginx::Time.local_time
 * Nginx::Time.parse_http_time
remove unnecessary mrbgems
add flags for mruby in configure

The flags mruby requires are described
in mruby/build/host/lib/libmruby.flags.mk.