Add support for Validating Session Token/Retrieving Current User

[jdmaresco]

Planning to work on an implementation of this in the next few weeks but if anyone wants to do it first, be my guest.

API reference: https://parse.com/docs/rest#users-validating
Relevant blog post about new REST API feature: http://blog.parse.com/2013/11/04/validating-session-tokens-with-the-rest-api/

Suggesting implementation of a User.me function...

[adelevie]

This would be great.

[dimitarvp]

I know this is not a constructive comment, but I want to see this implemented as well.

[jdmaresco]

I haven't used Parse for a a while, and I never got around to implementing this, sorry!