Add support for PLAINTEXT signature method

I was trying to authenticate to Freshbooks using _request_, and it turned out that they currently support **only** `PLAINTEXT` as a signature method http://developers.freshbooks.com/authentication-2/

So I read about it here http://oauth.net/core/1.0/#anchor22 and I figured that it should be the same as the `hmacsign` minus the base string.

Now obviously the first 3 arguments are not needed in this case, but I kept them here because I just pointed my request's oauth-sign dependency to my patched version and it **worked**

I can remove these 3 arguments and after this is being published I can patch the request's source code as well, or we can just leave it as it is. Let me know what do you think.

Also summoning @nylen