Add support for form parameters

[tjquinno]

This PR adds support for form parameters in the entity, either as plain text or as URL-encoded text.

Developers will register the new FormParamsSupport with their Routing which will subsequently allow them to use request.content().as(FormParams.class) to get convenient access to the form parameters. FormParams extends Parameters for consistency with retrieving other types of parameters.

This is not full form data support, which will probably need to wait for the multipart support.

Resolves #784

[tjquinno]

Thanks to Tomas for the review over zoom. The latest commit incorporates his suggestions.

Most notably, the new FormParamsImpl class now extends ReadonlyParameters (instead of basically duplicating that class). Although there is really no behavioral difference between a FormParamsImpl instance and a ReadonlyParameters instance I decided to keep the separate implementation class because that's a good place for the static regex parsing of the entity content and preparing the map that's passed to the constructor. There's nowhere else that is as good a location for that logic.

[tomas-langer]

LGTM