Implement basic text generation.

* bzrlib/tests/doc_generate/builders/test_texinfo.py:
(TestTexinfoFileGeneration.test_files_generated): Check formatting
of paragraphs and styled text.

* bzrlib/doc_generate/writers/texinfo.py:
(TexinfoTranslator.visit_paragraph)
(TexinfoTranslator.depart_paragraph): Put newlines around
paragraphs.
(TexinfoTranslator.visit_emphasis)
(TexinfoTranslator.depart_emphasis): Implement emphasis.
(TexinfoTranslator.visit_strong, TexinfoTranslator.depart_strong):
Implement strong.