fs_event: use cached Persistent syms instead

Instead of String::New every time, use a Persistent sym. This can be
accomplished in two ways:

1) Local<String> str = *persistent_str_sym;

2) Handle<String> str = persistent_str_sym;

I've chosen to use the latter method for simplicity's sake.

Other small changes include creating syms on Initialize and removing
unnecessary Local casting on return values.