Define StreamEventFlag constants if not present

Resolves pipobscure/fsevents#34
Resolves pipobscure/fsevents#33
Resolves pipobscure/fsevents#20
Define StreamCreateFlag constants if not present
Configure gyp to build only on mac

Credit to @andreaskern for originally authoring this fix
Resolves pipobscure/fsevents#32
Bump nan to ~1.3.0
Replace deprecated NanRawString with NanUtf8String