confluence parser: support for ac:structured-macro

See: https://confluence.atlassian.com/display/DOC/Confluence+Storage+Format+for+Macros

At least in version 5.3 confluence macros also use ac:structured-macro instead of ac:macro and ac:rich-text-body aside of ac:plain-text-body for the content.
The difference between ac:rich-text-body and ac:plain-text-body is that ac:rich-text-body can contain inline-formatting. (PlainTextBodyTagHandler can be used to handle both cases.)
No idea about the difference between ac:macro and ac:structured-macro, however - to me in both cases a MacroTagHandler seems to work pretty well.