Added new PostField class to allow header with simple fields

Hello,

I had a problem using Guzzle: I needed to make a "classic" multipart request. That means that I didn't want to make a "form post". And for now guzzle can only make multipart request like a form.

With my changes you can now add fields that doesn't have name. It will generate part without "Content-Disposition" header. Also you can add an header for a field, like that you can specify the type of your part or any other header you want.

Actually my changes are not perfect IMO but here are my motivation to change like that:
* Avoid modifying the logic of the current code
* Access of methods to avoid adding potential deprecations needs in the future

I think the test is complete but i'm not sure about that, please check it.

I hope you will like this PR :-) .