Add methods to extract and recreate access tokens
Add methods to extract and recreate access tokens

Currently the unsplash_rb gem doesn't provide public methods to extract
access tokens and recreate those tokens at a later time.  This makes
persisting the bearer token difficult.

This commit adds two methods to the connection class.  #extract_token
uses the OAuth2 gem's OAuth2::AccessToken#to_hash method to extract a
hash that can then be persisted for later use.  #create_and_assign_token
takes as a param the extract hash token, creates a new
OAuth::AccessToken object, and then assigns the new object to
@oauth_token.

CrewLabs/unsplash_rb,unsplash/unsplash_rb,unsplash/unsplash_rb,CrewLabs/unsplash_rb