Previous page count and page references need not be attributes
Support reStructuredText contents

- do not use the table of contents generated by docutils, but use
RinohType's TableOfContents
- extend TableOfContents to allow generating a local table of contents

TODO: Styling for local TOCs needs to be specified explicitly for each
level. Make indent_left relative to the parent TableOfContentsEntry?
Support reStructuredtext definition classifier

Some cleanup:
- adjust Definition for StyleSheet styling
- allow passing a style to CustomElement.process_content()
Allow setting the PDF metadata

- set the Creator field on instantiation of the PDF Document
- the document title and other fields can be passed on Document
instantiation or at a later point in time by writing to the
corresponding attributes
Change the cache extension to 'rtc' (RinohType Cache)
Nested enumerated section numbering (e.g. 1.1.1)

- rename number_separator to number_suffix
- number_separator is placed between the reference label of the parent
section and the number of this section
Include reStructuredText metadata in the PDF

docutils filters out the meta elements from the doctree, so this doesn't
work currently.