Add NanUnsafePersistent

It's very common to store `v8::Persistent` in STL containers which use `copy-value` style to store elements. However after node v0.11.x the `v8::Persistent` is not copiable nor assignable any more, the `NanUnsafePersistent` is just like `v8::Persistent` but can be stored in STL containers.

`NanPersistentToLocal` and `NanDispose` can still work on `NanUnsafePersistent`, and there are equivalent functions for operating `NanUnsafePersistent `: `NanInitUnsafePersistent` and `NanAssignUnsafePersistent`.