Simpler AttachmentToHTML::Adapters::PDF interface

TODO: We really should be testing the full output of PDF#body, but
inconsistencies between pdftohtml prevent sensible means of doing this.

For example:

    adapter.body.should == %Q(\n<A name=1></a>thisisthebody<br/>\n<hr>\n)

Fails because some versions (correctly!) use lower case tag names.
Simpler AttachmentToHTML::Adapters::RTF interface

TODO: We really should be testing the full output of RTF#body, but we currently
want to remain consistent with Adapters::PDF as many methods are shared between
the Adapters.

A more correct spec might be:

    expected = %Q(<font size=3><font color="#000000">thisisthebody</font></font>)
    adapter.body.should == expected
Simpler AttachmentToHTML::Adapters::GoogleDocsViewer interface
Simpler AttachmentToHTML::Adapters::CouldNotConvert interface
Update AttachmentToHTML for new View class
Remove redundant AttachmentToHTML::HTML
Dynamically inject AttachmentToHTML::View content
Minor change to FoiAttachment#body_as_html

AttachmentToHTML#to_html already returns the raw html now
Update gubbins to call AttachmentToHTML