Method that retrieves an oauth Request Token for 3-legged authentication

[bijanmmarkes]

The purpose of this pull-request is to use the existing infrastructure of this plugin to retrieve a request token from Twitter that can be used to generate 3-legged authentication flows. Some changes had to be made to the Request method in order to properly decode the response from twitter. (text/html)

I created a custom getRequestToken method to make it simple to retrieve an access token in array format that can then be passed back to the client side as JSON rather than a URL Encoded string.

• Reference to issue: https://twittercommunity.com/t/why-is-the-return-type-of-post-oauth-request-token-text-html-when-it-is-clearly-not-html/698

Links:

• https://developer.twitter.com/en/docs/authentication/api-reference/request_token
• https://developer.twitter.com/en/docs/apps/callback-urls
• https://developer.twitter.com/en/docs/authentication/api-reference/authorize

Changelog:

• Refactored the Request Method to decode and return an array from Twitter responses that are formatted as text/html and application/x-www-form-urlencoded - POST oauth/request_token.
• Added a method that utilizes the existing Request Method and retrieves an oauth request token from twitter to be used for 3-legged authentication.
• Revised Readme with examples on how to use the new method.
• Updated PhpDoc and some cleanup.