Add RESTful method support for html forms

This PR allows calls to `CHtml::form()`, `CHtml::beginForm`
`CActiveForm::$method` etc to accept verbs (methods) other than `post` and `get`,
e.g. `delete`, `put`, `patch` etc

This is implemented by detecting non `post`, `get`, converting to `post`
 and adding a hidden `_method` field, similar to how CSRF protection works.

On the server side, `_method` is already supported by `CHttpRequest`