Implement JSAPI token to OAuth token conversion.

This method implements the conversion (and signature validation) of token-containing cookies returned via LinkedIn's JSAPI to long-lived OAuth tokens. This conversion/validation is [detailed here](https://developer.linkedin.com/documents/exchange-jsapi-tokens-rest-api-oauth-tokens).

It is highly likely that OAuth provides a higher-level method than `request` that is more appropriate, but I chose to follow this route after `get_access_token` proved troublesome.

I have not written any tests for this and I don't have plans to. I don't expect this will be pulled as-is because of that, but am submitting a pull request so that anyone interested in using/improving this has a chance of finding it.

Usage example:


    client = LinkedIn::Client.new(api_key, api_secret)
    client.authorize_from_jsapi_cookie(cookies["linkedin_oauth_#{api_key}"])