Added IODEF support

Added basic support for IODEF (http://tools.ietf.org/html/rfc5070).

The code currently supports the `IODEF-Document` element and all of its attributes along with the following sub-elements, including all of their attributes:

* `Incident`
    * `IncidentID`
    * `ReportTime`
* `Assessment` 
    * `Impact`
* `Contact`
    * `ContactName`
    * `Email`

Support is provided for multiple elements where permitted by the specification (currently `Incident`, `Assessment`, `Impact`, `Contact`, `Email`).